Claire Roederer
email: claire.roederer@em-strasbourg.eu

Marc Filser
email: marc.filser@u-bourgogne.fr

e-Voicing an opinion on a brand Working paper EM Kit Workshop 16 th may 2011

First part : online practices of consumers "Markets are conversations". With this first suggestion and the 94 others that followed, the authors of the Cluetrain manifesto [START_REF] Levine | The cluetrain manifesto : the end of business as usual[END_REF] highlighted in 1999, the fundamental changes that the advent of Internet would generate in the manner of considering the relationship to markets and more generally speaking commercial exchanges. They portend in particular, the progressive replacement of traditional massmarketing tools by the generalisation of a new means of communication characterised by novel forms of conversation between consumers on one side and companies and their brands on the other.

Consumers expect greater transparency, authenticity, reactiveness, options and support by the company with regard to its responsibilities in relation to them and to society in general [START_REF] Myron | Customers in the cockpit (Editorial)[END_REF]. Thanks to Internet, consumers have the means to be treated not as mere numbers, but as individuals to be heard. In other words, thanks to information technologies, they can enjoy new forms of power/control.

Internet is, in fact, considered as an empowerment tool both in the field of consumer behaviour and in information systems, to the extent that it allows consumers to interact with the rest of the world at different levels (personal, dyad, group, community) [START_REF] Amichai-Hamburger | Internet empowerment[END_REF].

A good illustration of this phenomenon among others: if a consumer has an unpleasant experience with a brand, he is able to divulge this information no longer simply to his next door neighbor, but to numerous friends world-wide as well as those listening to him on social networks, with the possibility of even providing visual and audio traces.

Consumers are not all equal in relation to these new uses of Internet and to the potential power they wield. Computer savviness, the consumer's representation of technology [START_REF] Kozinets | Technology/ideology: how ideological fields influence consumers"technology narratives[END_REF], along with the cultural capital, where age certainly affects the use that he can make of this potential source of power and consequently his degree/level of empowerment.

However, the different ways in which this power available to the consumer can be expressed is to be found in new practices under the form of cyber-experiences, that we have defined in the following paragraph. The conceptual framework of the experience enables us to grasp the large variety (of practices) that the consumer has access to and can implement in cyber-space.

Definition of cyber-experiences.

Cyber experiences or on-line experiences can be defined as all the consumer experiences, i.e. interactions of persons x object x situation [START_REF] Punj | An Interaction Framework of Consumer Decision Making[END_REF] which generate significance for the persons experiencing them [START_REF] Filser | Le marketing de la production d"expérience : Statut théorique et implications managériales[END_REF]2008), whether such interactions are market or non-market related. Cyber experiences presuppose an individual-screen interaction, irrespective of the screen size (desk computer or portable, smartphone or tablets of the Ipad type) and can take place in any real physical place and concern any product or service category [START_REF] Kozinets | The field behind the screen: using netnography for marketing research in online communities[END_REF].

Different categories of cyber-experiences

Internet must not always be considered as the reproduction/simulation of the real world, but rather as an alternative environment accessible via a computer and which is both from a company's point of view :

-a distribution channel for goods and services -an advertising medium and from the consumer's point of view, essentially :

-a potential source of unlimited information, -a consumption area -an area to voice one's opinion One can distinguish : (1) strictly market cyber experiences ; (2) both market and nonmarket cyber experiences.

(1) Strictly market experiences

This type of interaction covers online shopping experiences [START_REF] Soopramanien | Conflicting attitudes and scepticism towards online shopping : the role of experience[END_REF], and correspond to shopping experiences [START_REF] Tauber | Why Do People Shop?[END_REF] in conventional shops/stores selling goods and services. By cutting the storage and distribution costs, Internet allows several companies to market products that would not be economically viable to propose in conventional stores, thus resulting in an almost unlimited increase in offer [START_REF] Anderson | The long tail: why the future of business is selling less for more[END_REF], as illustrated by the long tail theory, and transforming this distribution channel into a vast borderless supermarket from the consumer's point of view. Ranging from purchases on Amazon.com to hotel room reservations on Booking.com and downloading music from Itunes, these cyber experiences correspond to a real market exchange and not free of charge. In most cases, they involve both the consumer and the company, but Internet has also promoted CtoC sales with online auction sites such as eBay [START_REF] Peters | An exploratory investigation of problematic online auction behaviors : experiences of eBay users[END_REF].

(2) Market and non-market cyber experiences These include :

-the search for information experience that comes prior to the buying and consumption experience [START_REF] Arnould | Consumers[END_REF].

The search for information, which has become extremely intuitive through the use of engines such as Google, is one of the activities most shared by web users. We could even maintain that the search for information is the starting point of any cyber-experience.

By relating the history of Google in his book, The Search, [START_REF] Battelle | How Google and its rival rewrote the rules of business and transformed our culture[END_REF], considers that the process of seeking information constitutes a major cultural change transforming the manner of contemplating market exchange. Information search on the Web has, in fact, strong implications as regards the manner of consumption and indeed constitutes a novel source of power/control available to the consumer. [START_REF] Clemons | How information changes consumer behavior and how consumer behavior determines corporate strategy[END_REF] believes that the information the web user is likely to obtain with just a few clicks, affects his behaviour and consequently, all the variables in a classic marketing mix : the price becomes what the consumer is willing to pay, advertising is increasingly determined by what the consumers have to say about the product online (online usergenerated content) and product design corresponds to the most strongly expressed consumers' preferences (crowdsourcing) [START_REF] Clemons | How information changes consumer behavior and how consumer behavior determines corporate strategy[END_REF] in order to fill the gaps in the existing market offer.

-entertainment experiences : Internet could be considered as a source of entertainment, with unspecified borders, in which the web user can enjoy an infinite number of recreational and/or instructive experiences. [START_REF] Addis | Commentary : New technologies and cultural consumption : edutainment is born[END_REF] refers to this type of experience as edutainment experiences, a contraction of education and entertainment. These experiences, whether or not free of charge, cover a range of games, reading, podcasts accessible via Internet, and constitute a polymorphous offer that is almost unlimited and regularly updated. To this effect, it is worth noting the practices of numerous students who no longer watch television, but the same programmes, series (American) -through podcasts on their computers.

-content creation experiences. We use this term to designate all the interactions person x object x situations [START_REF] Punj | An Interaction Framework of Consumer Decision Making[END_REF] involving the production/circulation of content in chat rooms, forums, blogs, or on a wider scale, the presence more or less active on social networks like Facebook. The experiences of "voicing an opinion" and the resulting discourse could relate to any consumer/brand interaction (having taken place on the Internet or in real life), whether real or fictional.

These comprise experiences resulting in the production of a consumer generated content [START_REF] Fournier | Exploring the evolving terrain of consumer generated brand content[END_REF].

The main objectives of identified cyber experiences are represented on Figure 1. It can be stated that the search for information could be entertaining just as entertainment could be an enriching process/experience and a source of information. The categories bring to light the main objectives sought.

Figure 1 Cyber experiences classified according to main objective sought

Cyber-experiences involving brands

If we focus on cyber-experiences involving brands, two major categories of practices can be distinguished : (1) practices implying consumer generated content as regards the brand ; (2) practices that affect a phase of the decision making process involved in buying the brand (Figure 2).

Content generation for a brand

Consumer generated content for a brand can take different forms. The content could be a commentary, a recommendation, a rate, tag, comment, blog, tweet, friend [START_REF] Hardey | To spin straw into gold? New lessons from consumer-generated content[END_REF] with the purpose of providing information to others which then becomes e-Wom. But the content generated for the brand could go as far as the creation of an advertisement referred to as consumer generated advertising) (Campbell et al., 2011), the brand parody [START_REF] Fournier | The univited brand[END_REF] or the generation of ideas for the brand called crowdsourcing.

Its circulation

The online release of such discourse implies their accessibility to a potentially high number of consumers, a geographical de-compartmentalisation and the immediacy of this accessibility to the message. The individual can communicate on different levels: personal (consumer -> company ; dyadic (consumer a -> consumer b) ; in groups (consumer a -> a consumer group) ; in a community [START_REF] Füller | Consumer empowerment through internet-based co-creation[END_REF].

These different levels of communication correspond to different virtual areas (social networks like Facebook, forums, blogs, virtual communities or online communities. [START_REF] Kozinets | The field behind the screen: using netnography for marketing research in online communities[END_REF] distinguishes five types of online communities: (1) the boards (also called newsgroups, usegroups or usenet groups), organized around a specific product or a service ; (2) independent web pages or web rings (thematic links of the Web pages) ; (3) thematic mailing [START_REF] Kozinets | The field behind the screen: using netnography for marketing research in online communities[END_REF].

Its trace

Furthermore, the discourse put online, whatsoever its form (textual, visual…) can remain for an indefinite period (even permanent) on the Internet, and is materialized by a written/visual trace which continues to exist. Examples of compromising photos on Facebook that continue to be circulated endlessly, even if they do not necessarily concern a brand, illustrate this point.

Theorisation of e-WOM/e-Word of Mouse

Consumer to consumer communication is meant to have a strong impact on consumers" This new manner of operating puts emphasis on the consumer networks, online groups and communities.

Incorporation of the e-WOM phenomenon by companies

Among the different forms of voicing opinions by consumers on the Internet, the customer reviews used as sales pitch is undoubtedly the most frequent. Ferrero, for which the pages exist in an "organic" way that is with fan generated input.

The fact that a person is a fan does not mean that he is an actual user of the brand, nor that he is perfectly satisfied with it. Being a fan is sometimes the required entrance ticket to be able to express an opinion on the brand. The fan pages present potential risks for the brand image because of their viral aspect and easy referencing.

Opinion/Review control by other customers

Customer opinions and reviews can be regulated in different ways on the Internet. It is thus evident that voicing of opinions and comments by customers is subject to some form of regulation on the internet.

Professionalization of voicing opinions on the internet

Certain consumer bloggers have turned this practice, initially non-professional, into a profession. We can find such examples in blogs dedicated to fashion [START_REF] Kretz | Pixelize me!": a semiotic approach of self digitalization in fashion blogs[END_REF]. In this example, voicing of an opinion on the internet has turned professional and fully recognized as such by brands.

This type of professionalized opinion giving is part of WOMM strategies and adopts specific forms of expression. [START_REF] Kozinets | Networked narratives : understanding word-of-mouth in online communities[END_REF] identifies 4 types of expression underlying brand related reviews, given by established bloggers: evaluation, explanation, embracing, endorsement.

Consumer-generated advertising

Internet and less onerous software or even open source, enables consumers to produce not only comment but also videos posted on sites such as YouTube. The use of videos is one of the main activities of web users (Campbell et al., 2011). These activities could be considered as part of corporate strategies as in the case of participatory advertising. The example of the Doritos brand and the advertisements for Superbowl created by web users, that of the Nespresso film The Capsule, of which the end part has been written by web users illustrate this point. The consumer is invited to generate communication for a brand, but the overall advertising is monitored by the company.

There exist videos on brands over which the company has less control, often of a satiric nature [START_REF] Fournier | The univited brand[END_REF].

Consumer-generated ideas

Furthermore, the web user could also be involved in the development process of a new product through the use of tool kits on the web [START_REF] Fuchs | Customer empowerment in new product development[END_REF]. The consumer"s participation in developing a new product corresponds to a change in MAP (Manufacturer Active Paradigm) to CAP (Consumer Active Paradigm) [START_REF] Rohrbeck | Sourcing innovation from your customer : how multinational enterprises use web platforms for virtual customer integration[END_REF], which reveals a major transformation in the consumer"s traditional role.

Crowdsourcing

Likewise, crowdsourcing for a brand consists in seeking the opinions of web users for its products, values or an element of its communication. The outcry following the recent change in the GAP logo is an example of this mechanism of crowdsourcing. The company appeased web users by seeking their ideas for the logo [START_REF] Fournier | The univited brand[END_REF].

Practices affecting the buying process

These cyber experiences impact one or the other phases of the decision-making process when buying. It can be considered that the tools proposed by Internet are used by consumers for strategic purposes to optimize information on the product or even increase their power of negotiation in relation to the company and consequently to gain control or counter balance control attributed to the company.

Figure 4: Cyber-experiences affecting the buying process

The search for information

Online cyberexperiences involving the brand

Consumer generated content for a brand

Practices affecting the buying process

The search for information

Group purchasing for better purchasing

Using multi-channels against the company Using several community managers for better negotiations Consumers are not merely content to search for information, but also attach importance to selecting their sources. As revealed by a recent survey conducted by Lightspeed Research (WPP group), social networks that are very popular as a means of communication, do not, however, succeed in attracting online buyers in search for an opinion on a product. Most French web users prefer (72%) to resort to search engines, market sites such as Amazon (56%) or consumer associations (36%). The English prefer market sites (72%) followed by search engines (70%), whereas Germans primarily consult search engines (76%) and consumer associations (65%). We can thus see that consumers not only seek information, but prioritize their sources of information and that certain cultural differences do emerge in these efforts to optimise purchases.

Collective buying power

The group buying sites (for eg. discounteam.com) offer to negotiate attractive prices or enable web users to form groups on the site forum to negotiate prices independently for a group purchase of a same product.

Collective purchasing could have a cultural dimension if one considers this booming phenomenon in China (tuangou) (Wang et Zhao, 2009 ;[START_REF] Areddy | Chinese Consumers Overwhelm Retailers With Team Tactics[END_REF]. In keeping with this same principle of grouping of potential buyers using forums on Internet, the Chinese version of Team Purchase, continues with a face to face encounter in the store where consumer groups meet to lower prices and sometimes quite aggressively. (Wang and Zhao, 2009 ;[START_REF] Areddy | Chinese Consumers Overwhelm Retailers With Team Tactics[END_REF]. The consumers turn to Internet to gain power in numbers thus increasing their negotiation possibilities with the company. We are far from the poetic flash mob and this clearly reveals the use of collective buying power to negotiate prices or terms of sale.

Using multi-channels against the company

Comparing prices and shopping across channels on the web by users has become increasingly common for greater bargaining power against companies. Thus web users implement a strategy called «click then mortar then click », which after an active search for information on the web, consists in going to the store to negotiate prices and then threatening to buy online if the negotiation is unsuccessful.

Using several community managers to negotiate

Other strategies concern web users who identify several community managers in charge of one same brand and negotiate with each of them by resorting to a form of competition among companies and community managers, thus fragmenting the e-marketing strategy in place.

Consider the following verbatim : "I was disappointed with a purchase. After placing several calls to the hotline, and getting no response, I placed very negative comments on the Facebook page of the brand. Three minutes later, the community manager got in touch with me, and my problem was immediately solved". This example illuminates the opportunistic component of consumer e-voicing : the goal is not so much to inform other consumers than exerting pressure on the brand. And motives for such expressions on the net are not rooted in a radical criticism of the consumption society, but rather on strictly utilitarian motives… After a rapid overview of cyber-experiences steered by the consumer in relation to the brand, we will study the category of cyber-experiences involving the production of a discourse. In keeping with the finding of [START_REF] Fournier | The univited brand[END_REF], the grid (Figure 5) explores the elements potentially underlying the online opinion statements of consumers regarding a brand/company. This grid is structured around the context of voicing an opinion by characterizing it in relation to an element triggering the opinion statement (transaction vs. non transaction) on the one hand and characterizing the context by taking into account the consumer"s orientation (cooperative/conflictual). It results in four quadrants which we will analyze.

Second part: Research orientation

Analysis grid of the reasons and forms of voicing an opinion online

Among the different research themes of interest to us, we will focus on those suggested by [START_REF] Fournier | Exploring the evolving terrain of consumer generated brand content[END_REF], corresponding to quadrants 1 and 2 of figure 5.

Consumer-generated branded content

Nature and effectiveness of consumer-generated branded content [START_REF] Fournier | Exploring the evolving terrain of consumer generated brand content[END_REF].

Consumer as "translators" and "co-creators" of meaning: is this new role a source of value?

Market surveys show the high level of acceptance by consumers of information provided in the internet by other customers, and research has highlighted the role actually played by this kind of information in the consumer decision process. If the benefits of consumer generated information to other consumers are well documented, less attention has been devoted to the benefits that a consumer can find in this process of information generation. A reference to the opinion leadership literature is a first track of investigation, even though the e-opinion leader may capture significantly different forms of recognition in the internet, when compared with the "classical" forms of opinion leadership in a "real world" social circle.

Does CGA present a fundamentally different advertising paradigm or does it operate to persuade in the same way as company-sponsored advertising? The same market surveys signal a potential gap between the credibility of company sponsored information, and consumer generated information in the internet. Whereas banners, popups and other familiar communication supports are perceived as intrusive and generate negative comments, consumer generated information receives a systematically more positive assessment. How will these contrasted effects impact the role of different sources in the consumer decision process ? Will consumer generated content become a new source of consumer empowerment, and lead to a significant shift in the balance of power in the marketplace ? Or will the brands be able to keep control over those new means of expression ? These questions are decisive in order to better understand the future shape of the relations in the marketplace.

Active internet user behavior pattern overtime

There is a strong need to investigate the link between consumer expression on the web and consumer behavior. For example, are very "loud" consumers more loyal to the brand over time than less active customers ? The extreme case of the brand tribe has been analyzed by European and north American field works, and exemplifies a very intense link between voice and action. But it is dubious to consider that every consumer posting information on the web is a passionate member of the brand community.

A parallel may be drawn with the results of research exploring the link between satisfaction / dissatisfaction and brand loyalty. Some dissatisfied consumers may become very loyal buyers of the brand if the motives of their dissatisfaction have been solved by the brand. Do such traits operate on the web ? Maybe a dissatisfied consumer will post information on a forum, get involved in an intense exchange of information with other customers, and revise his former negative attitude toward the brand, leading perhaps to the diffusion of more positive information later.

Using longitudinal data on e-voicing, and linking them with actual buying behavior, could provide useful cues to better assess links between speech and action.

What variables explain the tendency to voicing an opinion on internet

4.Firms ' stances in reaction to internet users voices

Finally, this research should address a managerial question : how should a company take into account a typology of internet users based on "voicing styles" ? Should this variable be measured (and how ?) and integrated in consumer databases ? The recognition of the critical role of consumer involvement in information processing has led brands to radically contrasted persuasive strategies when they address low or high involvement segments. Should voicing styles be taken into account in the same way ? And how effective (and efficient) are different answer strategies ?

To conclude, we would like to stress the radical change that is occurring in the marketplace due to the development of consumer expression in the internet. Theoretical analyses of this behavior might be located along a continuum. On one hand, e-voicing is reflecting a massive rejection of the consumption society, and a sign of some kind of consumer revolution of the "reclaim the streets" style. Such forms of expressions probably exist. But on the other hand, evoicing may reflect a basic need of the contemporary consumer to get in touch with other people, and to be considered for some kind of expertise he is detaining. And between those extremes, we could imagine a large variety of motives, some basically utilitarian and opportunistic, some more altruistic in essence, reflecting an authentic willingness to share the knowledge and the experience with others.

This research project should rely on a large variety of theoretical frames, from an individualistic psychological frame, to broader schemes derived from the Consumer Culture Theory. And before empirical measures are developed to attempt consumer typologies, a massive qualitative exploratory research should be led to give more substance to the research propositions formulated in this paper.

Figure 2 :

 2 Figure 2 : Cyber-experience categories involving the brand

.Figure 3

 3 Figure 3 Cyber-experiences including consumer generated content for a brand

  buying behavior. Longtime understood as face to face communication, online communication from the consumer has taken multiple forms with the development of Internet[START_REF] Gupta | How e-WOM recommendations influence product consideration and quality of choice: A motivation to process information perspective[END_REF] and one can talk about e-Word of Mouth as Word of mouse, since this high tech version of ancestral word to mouth communication is now taken over by the computer mouse.From a fugitive oral form we now have a written and visual form that lasts and can be transmitted effortlessly.[START_REF] Kozinets | Networked narratives : understanding word-of-mouth in online communities[END_REF] identify three phases in the evolution of models theorizing WOM : (1) the model of organic inter-consumer influence (2) the model of linear influence ; (3) the model of joint generation via a network. WOM was theorized as early as the forties of the last century as an influential model among consumers.[START_REF] Kozinets | Networked narratives : understanding word-of-mouth in online communities[END_REF] qualify this first model as organic to the extent that it functions without intervention from marketers. Thereafter, marketing theoreticians and practitioners recognized the importance of being able to identify influential consumers in the WOM process to attempt to associate opinion leaders, referred to as the linear influence model. Lastly, to the advantage of the development of Internet, marketers became interested in another manner of steering WOM.

Fournier

  and Avery (2011) speak of Open Source Branding in relation to the observable mechanisms/practices on Internet which involve the web user and the brand. After having identified four themes underlying these practices (the age of social collective, the age of transparency, the age of criticism, the age of parody) and three lines of corporate behavior in response (The path of least resistance, Playing their game, Leveraging Web 2.0 interconnectedness), Fournier and Avery contend that observed practices call into question the paradigm of branding management.

Figure 5 :

 5 Figure 5: Grid for structuring forms of online opinion statements

  Most multi-brand distribution sites presently have a mechanism enabling them to take into account customer reviews in one way or the other. By way of example, ASICS has chosen Bazaarvoice to incorporate customers" opinions on its 30 sites across Europe and on Facebook. The brand can henceforth control this type of content generated by users and integrate it in all aspects of its business, from product design to marketing (source : Kovacs e-marketing portal, 22/03/2011).Customer opinion can be received in different areas. The fan pages on Facebook constitute an area that is increasingly sought by brands. With over 600 million active members and an interface translated into over 70 languages, Facebook has become a powerful communication

	tool	with	the	public,	partners,	customers	and	potential	employees.

(http://www.esens.unige.ch/ consulted 20/4/2011). Vanksen has just published a white paper on the phenomenon of "fan pages" on Facebook. According to this source, among the brands having the greatest number of fans on Facebook are Ferrero, Mars and Apple. The most represented sectors are the agribusiness, fashion and internet. Mars, the second most present brand on Facebook, has a community manager providing input for the fan page, contrary to

INFORMATION SEARCH


Pierre Riteau
email: pierre.riteau@irisa.fr

Christine Morin

Thierry Priol
email: thierry.priol@inria.fr

Shrinker: Improving Live Migration of Virtual Clusters over WANs with Distributed Data Deduplication and Content-Based Addressing

Keywords: Virtualization, Live Migration, Wide Area Networks, Cloud Computing

Live virtual machine migration is a powerful feature of virtualization technologies. It enables efficient load balancing, reduces energy consumption through dynamic consolidation, and makes infrastructure maintenance transparent to users. While live migration is available across wide area networks with state of the art systems, it remains expensive to use because of the large amounts of data to transfer, especially when migrating virtual clusters rather than single virtual machine instances. As evidenced by previous research, virtual machines running identical or similar operating systems have significant portions of their memory and storage containing identical data. We propose Shrinker, a live virtual machine migration system leveraging this common data to improve live virtual cluster migration between data centers interconnected by wide area networks. Shrinker detects memory pages and disk blocks duplicated in a virtual cluster to avoid sending multiple times the same content over WAN links. Virtual machine data is retrieved in the destination site with distributed content-based addressing. We implemented a prototype of Shrinker in the KVM hypervisor and present a performance evaluation in a distributed environment. Experiments show that it reduces both total data transferred and total migration time.

Introduction

The complete encapsulation of execution environments (applications combined together with their underlying OS) in virtual machines (VMs) allowed the development of live virtual machine migration [START_REF] Clark | Live Migration of Virtual Machines[END_REF][START_REF] Nelson | Fast Transparent Migration for Virtual Machines[END_REF]. This mechanism relocates a virtual machine from one host to another with minimal downtime, usually not noticeable by users. This offers numerous advantages for data center management, including:

Load balancing: VMs can be dynamically migrated depending on workload, offering more efficient usage of computing resources.

Reduced energy consumption: VMs with low workloads can be consolidated to fewer physical machines, making it possible to power off nodes to reduce energy consumption.

Transparent infrastructure maintenance: Before physical machines are shut down for maintenance, administrators can relocate VMs to other nodes without noticeable interruption of service for users.

Mainstream hypervisors usually do not support live migration between different data centers connected with wide area networks (WANs). To avoid transferring persistent state of VMs, which can be of large size (from hundreds of megabytes to dozens of gigabytes and beyond), they depend on shared storage, usually not accessible across different data centers. They also require that migrated VMs stay in the same local network, to keep network connections uninterrupted after migration to new host machines.

These restrictions prevents users from getting the benefits of live virtual machine migration over WANs. It would allow administrators to load balance workload between several data centers, and offload VMs to other sites whenever a site-wide maintenance is required. Users with access to private clouds (private computing infrastructures managed with cloud computing stacks) could seamlessly migrate VMs between private and public clouds depending on resource availability. It could also allow them to leverage variable price between competing cloud providers.

Fortunately, state of the art systems allow to use live migration over WANs by migrating storage [START_REF] Bradford | Live wide-area migration of virtual machines including local persistent state[END_REF][START_REF] Luo | Live and incremental whole-system migration of virtual machines using block-bitmap[END_REF][START_REF] Hirofuchi | A Live Storage Migration Mechanism over WAN for Relocatable Virtual Machine Services over Clouds[END_REF][START_REF] Wood | CloudNet: Dynamic Pooling of Cloud Resources by Live WAN Migration of Virtual Machines[END_REF] and network connections [START_REF] Bradford | Live wide-area migration of virtual machines including local persistent state[END_REF][START_REF] Harney | The Efficacy of Live Virtual Machine Migrations Over the Internet[END_REF][START_REF] Wood | CloudNet: Dynamic Pooling of Cloud Resources by Live WAN Migration of Virtual Machines[END_REF]. However, the large amounts of data to migrate make live migration over WANs expensive to use, especially when considering migrations of virtual clusters rather than single VM instances.

Previous research showed that VMs running identical or similar operating systems have significant portions of their memory and storage containing identical data [START_REF] Waldspurger | Memory resource management in VMware ESX server[END_REF][START_REF] Gupta | Difference Engine: Harnessing Memory Redundancy in Virtual Machines[END_REF]. In this paper, we propose Shrinker, a live virtual machine migration system leveraging this common data to improve live virtual cluster migration between data centers interconnected by wide area networks. Shrinker detects memory pages and disk blocks duplicated in a virtual cluster to avoid sending multiple times the same content over WAN links. Virtual machine data is retrieved in the destination site with distributed content-based addressing. We implemented a prototype of Shrinker in the KVM hypervisor [START_REF] Kivity | kvm: the Linux Virtual Machine Monitor[END_REF] and present a performance evaluation in a distributed environment. Our experiments show that it reduces both total data transferred and total migration time. This paper is organized as follows. Section 2 presents related work. Section 3 covers the architecture of Shrinker. Section 4 describes our prototype implementation in the KVM hypervisor, presents our experiments and analyzes the results. Finally, Section 5 concludes and discusses future work.

Background and Related Work

Live virtual machine migration [START_REF] Clark | Live Migration of Virtual Machines[END_REF][START_REF] Nelson | Fast Transparent Migration for Virtual Machines[END_REF] has traditionally been implemented with pre-copy [START_REF] Theimer | Preemptable remote execution facilities for the V-system[END_REF] algorithms. Memory pages are transferred to the destination host while the virtual machine is still executing. Live migration continues sending modified memory pages until it enters a phase where the virtual machine is paused, the remaining pages are copied, and the virtual machine is resumed on the destination host. This phase is responsible for the downtime experienced during live migration, which should be kept minimal. This downtime can range from a few milliseconds to seconds or minutes, depending on page dirtying rate and network bandwidth [START_REF] Akoush | Predicting the Performance of Virtual Machine Migration[END_REF].

Numerous works have been proposed to improve live migration and optimize its performance metrics: total data transferred, total migration time, and downtime. They can be classified in two types: optimizations of the pre-copy approach, and alternatives to pre-copy. Optimizations include compression, delta page transfer, and data deduplication. Compression is an obvious method to reduce the amount of data transferred during live migration. Jin et al. [START_REF] Jin | Live Virtual Machine Migration with Adaptive Memory Compression[END_REF] use adaptive compression to reduce the size of migrated data. Their system chooses different compression algorithms depending on memory page characteristics. Delta page transfer [START_REF] Hacking | Improving the live migration process of large enterprise applications[END_REF][START_REF] Wood | CloudNet: Dynamic Pooling of Cloud Resources by Live WAN Migration of Virtual Machines[END_REF][START_REF] Svärd | Evaluation of Delta Compression Techniques for Efficient Live Migration of Large Virtual Machines[END_REF] optimizes the transmission of dirtied pages by sending difference between old pages and new pages, instead of sending full copies of new pages. Data deduplication [START_REF] Zhang | Exploiting Data Deduplication to Accelerate Live Virtual Machine Migration[END_REF][START_REF] Wood | CloudNet: Dynamic Pooling of Cloud Resources by Live WAN Migration of Virtual Machines[END_REF] detects identical data inside the memory and disk of a single virtual machine and transfers this data only once.

An alternative to pre-copy is live migration based on post-copy [START_REF] Hines | Post-copy based live virtual machine migration using adaptive pre-paging and dynamic self-ballooning[END_REF][START_REF] Hirofuchi | Enabling Instantaneous Relocation of Virtual Machines with a Lightweight VMM Extension[END_REF], which first transfers CPU state and resumes the VM on the destination host. Memory pages are then fetched from the source host on demand, in addition to a background copying process to decrease the total migration time and quickly remove the residual dependency on the source host. Similarly, SnowFlock [START_REF] Lagar-Cavilla | SnowFlock: rapid virtual machine cloning for cloud computing[END_REF] uses demand-paging and multicast distribution of data to quickly instantiate VM clones on multiple hosts. Another alternative was proposed by Liu et al. [START_REF] Liu | Live migration of virtual machine based on full system trace and replay[END_REF], based on checkpointing/recovery and trace/replay. By recording non-deterministic events and replaying them at the destination, live migration efficiency is greatly improved. However, their approach is not adapted to migrate SMP guests.

All these systems were designed in the context of live migration of single VMs, and do not take advantage of data duplicated across multiple VMs. However, previous research [START_REF] Waldspurger | Memory resource management in VMware ESX server[END_REF][START_REF] Gupta | Difference Engine: Harnessing Memory Redundancy in Virtual Machines[END_REF][START_REF] Wood | Memory Buddies: Exploiting Page Sharing for Smart Colocation in Virtualized Data Centers[END_REF][START_REF] Milos | Satori: Enlightened Page Sharing[END_REF] has shown that multiple VMs have significant portions of their memory containing identical data. This identical data can be caused by having the same versions of programs, shared libraries and kernels used in multiple VMs, or common file system data loaded in buffer cache. This identical data can be leveraged to decrease memory consumption of colocated VMs by sharing memory pages between multiple VMs [START_REF] Waldspurger | Memory resource management in VMware ESX server[END_REF][START_REF] Gupta | Difference Engine: Harnessing Memory Redundancy in Virtual Machines[END_REF][START_REF] Wood | Memory Buddies: Exploiting Page Sharing for Smart Colocation in Virtualized Data Centers[END_REF][START_REF] Milos | Satori: Enlightened Page Sharing[END_REF]. The same observation has been made for VM disks [START_REF] Partho | Design tradeoffs in applying content addressable storage to enterprise-scale systems based on virtual machines[END_REF][START_REF] Rhea | Fast, inexpensive content-addressed storage in foundation[END_REF][START_REF] Liguori | Experiences with Content Addressable Storage and Virtual Disks[END_REF], which is exploited to decrease storage consumption. To our knowledge, Sapuntzakis et al. [START_REF] Sapuntzakis | Optimizing the migration of virtual computers[END_REF] were the first to leverage identical data between multiple VMs to improve migration. However, their work predates live migration and supported only suspend/resume migration, where the VM is paused before being migrated. Additionally, they only took advantage of data available on the destination node, limiting the possibility of finding identical data. A similar approach was also proposed by Tolia et al. [START_REF] Tolia | Using Content Addressing to Transfer Virtual Machine State[END_REF].

To allow live migration of VMs over WANs, two issues need to be solved: lack of shared storage and relocation to a different IP network. Storage can be migrated with algorithms similar to memory migration [START_REF] Bradford | Live wide-area migration of virtual machines including local persistent state[END_REF][START_REF] Luo | Live and incremental whole-system migration of virtual machines using block-bitmap[END_REF][START_REF] Hirofuchi | A Live Storage Migration Mechanism over WAN for Relocatable Virtual Machine Services over Clouds[END_REF][START_REF] Wood | CloudNet: Dynamic Pooling of Cloud Resources by Live WAN Migration of Virtual Machines[END_REF]. Network connections can be kept uninterrupted after migration using tunneling [START_REF] Travostino | Seamless live migration of virtual machines over the MAN/WAN[END_REF][START_REF] Bradford | Live wide-area migration of virtual machines including local persistent state[END_REF], VPN reconfiguration [START_REF] Wood | CloudNet: Dynamic Pooling of Cloud Resources by Live WAN Migration of Virtual Machines[END_REF] or Mobile IPv6 [START_REF] Harney | The Efficacy of Live Virtual Machine Migrations Over the Internet[END_REF]. The architecture of Shrinker supports storage migration. Additionally, since Shrinker focuses solely on optimizing data transmission during live virtual cluster migrations over WAN, it is compatible with all solutions for keeping network connections uninterrupted. Therefore, we do not discuss further this issue, as it is out of scope of this paper.

Architecture of Shrinker

We propose Shrinker, a system that improves live migration of virtual clusters over WANs by decreasing total data transferred and total migration time. During live migration of a virtual cluster between two data centers separated by a WAN, Shrinker detects memory pages and disk blocks duplicated among multiple virtual machines and transfers identical data only once. In the destination site, virtual machine disks and memory are reconstructed using distributed content-based addressing. In order to detect identical data efficiently, Shrinker leverages cryptographic hash functions. These functions map blocks of data, in our case memory pages or disk blocks, to hash values of fixed size, also called digests. These functions differ from ordinary hash functions because they are designed to render practically infeasible to find the original block of data from a hash value, modify a block of data while keeping the same hash value, and find two different blocks of data with the same hash value. Using hash values to identify memory pages and disk blocks by content is interesting because hash values are much smaller than the data they represent. For instance, a 4 kB memory page or disk block is mapped to a 20 bytes hash value using the SHA-1 [19] cryptographic hash function, a size reduction of more than 200 times.

We first present the architecture of Shrinker, and then discuss security considerations caused by our use of cryptographic hash functions.

Architecture Overview

In a standard live migration of a virtual cluster composed of multiple VMs running on different hypervisors, each live VM migration is independent. VM content is transferred from each source host to the corresponding destination host, with no interaction whatsoever between the source hypervisors or between the destination hypervisors. As a consequence, when migrating a virtual cluster over a WAN, data duplicated across VMs is sent multiple times over the WAN link separating the source and destination hypervisors.

To avoid this duplicated data transmission, Shrinker introduces coordination between the source hypervisors during the live migration process. This coordination is implemented by a service, running in the source site, that keeps track of which memory pages and disk blocks have been sent to the destination site. Before sending a memory page or a disk block, a source hypervisor computes the hash value of this data and queries the service with this hash value. If no memory page or disk block with the same hash value has previously been sent to the destination site, the service informs the hypervisor that it transfer the data. The service also registers the hash value in its database. Later, when the service receives a subsequent query for another memory page or disk block with the same hash value, it informs the querying hypervisor that the data has already been sent to the destination site. Based on this information, the hypervisor sends the hash value of the data to the destination host instead of the real content. This mechanism essentially performs data deduplication by replacing duplicated transmissions of memory pages and disk blocks by transmissions of much smaller hash values. Figure 1 illustrates a live migration between two pairs of hypervisors, with a cryptographic hash function creating 16-bit hash values. Source node 1 has to send a memory page or disk block identified by the hash value a0f3. Instead of sending it directly like in a standard live migration, it first queries the coordination service (step 1). Since this is the first time that this data has to be sent to the destination site, the service informs source node 1 that it should send the data. It also updates its internal database to keep track of this hash value (step 2). After receiving the answer from the coordination service, source node 1 sends the data to destination node 1 (step 3). Afterwards, source node 2 queries the service for the same data (step 4). The service informs source node 2 that this data has already been sent to a node in the destination site (step 5), which prompts source node 2 to send the hash value a0f3 (2 bytes) instead of the full content (4 kB) to destination node 2 (step 6).

Since destination nodes receive a mix of VM data (memory and disk content) and hash values from source nodes, they need to reconstruct the full VM content before the VM can be resumed. This is where distributed content-based addressing is used. When a full memory page or disk block is received by a destination node, its hash value and the IP of the node are registered into an indexing service, running in the destination site. When destination nodes receive hash values from source nodes, they query the indexing service to discover a node that has a copy of the content they are requesting. After contacting a node and receiving a copy of the content, they register themselves in the indexing service for this data, which increases the number of hosts capable of sending this data to another hypervisor. This process is illustrated in Fig. 2. As in Fig. 1, destination node 1 receives a memory page or disk block identified by hash a0f3 (step 1). It registers itself as having a copy of this data in the indexing service (step 2). Afterwards, as in Fig. 1, source node 2 sends hash value a0f3 to destination node 2 (step 3). In order to get the data identified by this hash value, destination node 2 queries the indexing service to discover a host which has a copy of the corresponding content (step 4). The indexing service informs it that destination node 1 has a copy of this data (step 5). Destination node 2 asks destination node 1 for this content (step 6), receives it (step 7), and registers itself in the indexing server as having a copy (step 8).

Since the live migration is performed while the virtual machine is still executing, it is possible that data that was sent to the destination site has since been overwritten and is not accessible in any destination hypervisor. All destination hypervisors open a communication channel with their corresponding source hypervisor. This channel can be used to directly request data to the source node.

Security Considerations

The possible number of different 4 kB memory pages and disk blocks (2 4096 ) is bigger than the number of possible hash values (2 160 for SHA-1). As a consequence, the use of cryptographic hash functions opens the door to collisions: it is theoretically possible that memory pages and disk blocks with different content map to an identical hash value. However, the properties offered by cryptographic hash functions allow us to use these hash values with a high confidence. The probability p of one or more collisions occurring is bounded by [START_REF] Akoush | Predicting the Performance of Virtual Machine Migration[END_REF], where n is the number of objects in the system and b the number of bits of hash values [START_REF] Quinlan | Venti: A New Approach to Archival Storage[END_REF]:

p ≤ n(n -1) 2 × 1 2 b . (1)
If we consider a very large virtual cluster consisting of 1 exabyte (2 60 bytes) of 4 kB memory pages and disk blocks migrated by Shrinker using the SHA-1 hash function, the collision probability is around 10 -20 . This is considered to be much less than other possible faults in a computing system, such as data corruption undetectable by ECC memory. However, [START_REF] Akoush | Predicting the Performance of Virtual Machine Migration[END_REF] gives the probability of an accidental collision. Although the theoretical number of operations to find a collision is approximately 2 n 2 (birthday attack), attackers can exploit weaknesses of the hash function algorithm to find collisions more easily. For example, researchers have shown attacks against SHA-1 that decrease the number of operations to find collisions from 2 80 to 2 69 [START_REF] Wang | Finding Collisions in the Full SHA-1[END_REF]. Assuming that finding collisions is possible, an attacker capable of storing arbitrary data in VMs (for instance, by acting as a client interacting with web servers) could inject colliding data in these VMs. After migrating them with Shrinker, memory content would be corrupted because two different pages would have been replaced by the same data. If such attacks become practically feasible in the future, Shrinker can use stronger cryptographic hash functions, such as those from the SHA-2 family. Even though these hash functions are more computationally expensive, Shrinker remains interesting as long as the hash computation bandwidth is larger than the network bandwidth available to each hypervisor.

Implementation & Performance Evaluation

In this section, we first describe the implementation of our Shrinker prototype. Then, we present and analyze the results of our experiments performed on the Grid'5000 testbed.

Implementation

We implemented a prototype of Shrinker in the KVM hypervisor [START_REF] Kivity | kvm: the Linux Virtual Machine Monitor[END_REF]. KVM is divided in two main parts. The first part is composed of two loadable kernel modules providing the core virtualization infrastructure. The second part is a modified version of QEMU [START_REF] Bellard | QEMU, a Fast and Portable Dynamic Translator[END_REF] running in user space to provide higher level features, including virtual device emulation and live migration. This prototype is fully implemented in the user space component of KVM version 0.14.0-rc0 and is about 2,000 lines of C code. We use Redis [START_REF] Sanfilippo | Redis[END_REF] version 2.2.0-rc4, a high performance key-value store, to implement the coordination and indexing service. Additional dependencies are OpenSSL, used to compute hash values, Hiredis, a C client library for Redis, and libev, a high-performance event loop library. Support for data deduplication of storage migration in our prototype is not yet fully finalized. As such, in the following experiments, we use the storage migration mechanism of KVM, with copy-on-write images. The use of copy-onwrite images allows KVM to send only storage data that has been modified since the boot of the VM, minimizing the amount of storage data to migrate. We expect our finalized prototype to offer even greater amounts of data deduplication for storage than for memory, since VM disks typically present large amounts of identical data, as shown in previous work (c.f. Sec. 2).

As explained in Sec. 3, before sending a memory page, source hypervisors need to contact the coordination service to know if the same page as already been sent to the destination site. Performing this query in a sequential manner would drastically reduce the live migration throughput (because of the roundtrip time between source hypervisors and the coordination service). To overcome this problem, our implementation performs these queries in a pipelined manner. Queries for multiple memory pages are sent in parallel, and the decision of sending the full content or the hash is made when the answer is received. The same method is used on the destination site for hypervisors to get memory content. The coordination service is implemented using the Redis SETNX command, with a memory page hash as key. If the hash is not already known by the service, it is registered and the return value notifies the source hypervisor that it was the first to register it. Otherwise, no change is performed in the service and the source hypervisor is notified that the hash was already registered. The indexing service is implemented using the set data structure of Redis. For each registered memory page, there is one corresponding set which holds information about hypervisors having a copy of the page. When destination hypervisors register a memory page, they send a SADD command with the page hash as key and an IP/port pair as value. When other destination hypervisors need to get a page, they send a SRANDMEMBER command, which selects a random hypervisor in the set corresponding to the queried page and returns its IP/port information. After connecting on this IP and port, they query the content of the page. Finally, when a destination hypervisor doesn't hold any more copy of a page, it unregisters it with a SREM command.

Evaluation Methodology

All the experiments presented in this paper are run on the paradent cluster of the Grid'5000 site of Rennes. We use Carri System CS-5393B nodes supplied with 2 Intel Xeon L5420 processors (each with 4 cores at 2.5 GHz), 32 GB of memory, and Gigabit Ethernet network interfaces. Physical nodes and VMs use the AMD64 port of Debian 5.0 (Lenny) as their operating system, with a 2.6.32 Linux kernel from the Lenny backports repository. VMs are configured to use 1 GB of memory. Two instances of Redis are running, each on a dedicated node.

To study the performance of our prototype, we configured a dedicated node to act as a router between source and destination hypervisors. Using the netem Linux kernel module, this router emulates wide area networks with different bandwidth rates. The emulated latency is set to a round trip time of 20 ms.

For our performance evaluation, we use a program from the NAS Parallel Benchmarks (NPB) [START_REF]NAS Parallel Benchmarks[END_REF], which are derived from computational fluid dynamics applications. We evaluate performance metrics of live migration during the execution of this workload. We measure total migration and total data transmitted by live migration. During our experiments, we discovered that the downtime caused by a live migration in KVM was overly important because of a miscalculation of the available bandwidth. This is why we do not report numbers for downtime, as they may be not representative of a correct behavior. We are investigating the issue and will report it to the KVM community. First, we observe that, whatever the bandwidth, the same amount of data is transmitted for both bandwidth rates. However, we can see that Shrinker sends much less data over the WAN link than the standard KVM migration. This allows Shrinker to reduce the total migration time, for instance from one minute to 20 seconds for the 500 Mbps link. Note that in the regular KVM live migration protocol, memory pages containing only identical bytes are already compressed and sent efficiently. As such, the improvement showed by Shrinker come from real data deduplication, and not from deduplication of zero pages.

Performance Results

Conclusion

In this paper, we presented the design and prototype implementation of Shrinker, a system that improves live migration of virtual clusters over WANs by decreasing total data transferred and total migration time. Shrinker detects memory pages and disk blocks duplicated among multiple virtual machines to transfer identical data only once. Virtual machine data is retrieved in the destination site with distributed content-based addressing. We implemented a prototype of Shrinker in the KVM hypervisor. Our experiments show that it reduces both total data transferred and total migration time.

In the future, we will finalize our Shrinker prototype to perform data deduplication on storage migration and evaluate its performance. We also plan to study how Shrinker can allow destination hypervisors to fetch data from local VM image repositories found in most cloud computing infrastructures to further decrease the amount of data sent over WAN links.

3

 3

Fig. 1 .

 1 Fig. 1. Distributed data deduplication used in Shrinker to avoid sending duplicate data on a WAN link

Fig. 2 .

 2 Fig. 2. Distributed content-based addressing used in Shrinker to reconstruct VM content on destination nodes

Figure 3 ( 16 Fig. 3 .

 3163 Figure 3(a) shows the total migration time of a 16 VMs cluster executing the ep.D.16 program, for two different bandwidth rates. Figure 3(b) shows the total amount of transmitted data during the live migration of these 16 VMs, for the same two bandwidth rates.

Note that this small hash value size is chosen to improve readability of the figure. In a real scenario, Shrinker can not use such hash function, since 65,536 different hash values would likely create collisions even for virtual machines of moderate sizes.

Acknowledgments. Experiments presented in this paper were carried out using the Grid'5000 experimental testbed, being developed under the INRIA AL-ADDIN development action with support from CNRS, RENATER and several Universities as well as other funding bodies (see https://www.grid5000.fr).


Yves Ousten

Isabelle Bord

Cyril Blot

Bruno Levrier

Laurent Bechou

Philippe Kertesz

Philippe Kertez

Environmental conditions influence on embedded capacitors -Comparison with discrete capacitors

come

EMBEDDED

CAPACITORS BASED ON POLYMER-CERAMIC DIELECTRIC

Embedded capacitors are made of a dielectric material layer between two metal layers (usually copper foils), buried in a PCB. Inorganic dielectric materials are partly replaced by organic polymers. Polymer-ceramic composites are expected to be the major candidate as dielectric interlayer for embedded capacitors because such composite materials present both characteristics of the processibility of polymers and a high dielectric constant (k) of inorganic materials as titanates [3,4,5]. Our study on embedded passive components is already in progress with different partners in various programs.

During this first study, such a commercial dielectric composite was studied.

The organic polymer is filled with high dielectric constant particles (BaTiO 3 according to EDX analysis presented Figure 1) modifying the epoxy film.

ENVIRONMENTAL CONDITIONS INFLUENCE

In order to perform a first robustness study, the impact of temperature and humidity on embedded capacitors based on a polymer-ceramic material was successively studied.

1.TEMPERATURE IMPACT ON CAPACITANCE

Capacitance measurements versus frequency were carried out using the impedance meter HP 4192A and the test fixture HP 16034E in the frequency range [100Hz -10MHz].

Figure 2 shows the capacitance variation versus frequency in the temperature range [25-150°C]. The electrode area of the studied capacitor is 6.0x6.0mm² and the dielectric thickness is 16µm. Whatever the temperature, the capacitance decreases with frequency due to the decrease of the relative permittivity r ( )

ε ω with frequency. The capacitance C( ) ω is linked to r ( ) ε ω by the following equation:

( ) ( ) 0 r S C d ε ω ε ω = (1)
Where ω is the excitation frequency, ε 0 the vacuum permittivity, S the electrode area and d the dielectric thickness.

Moreover the capacitance increases with temperature due to the increase of the relative permittivity r ( ) ε ω with temperature. A change of the behaviour of the capacitance is observed after 125°C because of the glass transition of the material (Tg close to 120 °C).

For robustness study, another commercial polymer-ceramic material is considered (also based on BaTiO3). The electrode area of the studied capacitors is 1.0x1.0mm² and the dielectric thickness is 8µm. Whatever the temperature, the capacitance decreases with exposure time. At ambient temperature, the dielectric material contains a little part of water. After exposure to 85°C / 150°C, water is evaporated. Water permittivity is higher than material one whatever the frequency. So the material permittivity and then the capacitance decrease after exposure to 85°C / 150°C.

The behaviour is similar at 85°C and 150°C. However the capacitance decrease is accelerated at 150°C: the same variation is observed at 150°C after 80h and at 85°C after 155h.

Figure 4 presents the capacitance variation versus frequency after 1000h at 150°C after drying. After drying, the capacitance significantly decreases due to water evaporation. A maximal capacitance variation of only 7% is observed at high frequency after 1000h.

At high frequency, a gradual slope change is observed with exposure time. This may correspond to a modification of the material during thermal ageing.

MOISTURE IMPACT ON CAPACITANCE

Capacitance measurements versus frequency were carried out after various exposure times to 85°C / 85% of humidity.

Figure 5 presents the capacitance variation versus frequency for the same embedded capacitor as previously (electrode area 1.0x1.0mm² and dielectric thickness close to 8µm).

Figure 5: Capacitance Vs frequency after various exposure times to 85°C / 85% of humidity.

Whatever the dielectric material, capacitance increases with exposure time because of water absorption by the material (water permittivity higher than polymer-ceramic permittivity).

After 138 hours, no significant variation is observed.

DISCRETE CERAMIC CAPACITORS

During a precedent FP6 program (AMELIE), we have tested two lots of 25 X7R 0805 ceramic capacitors (2,03x1,27x1 mm +/-0.2 mm) 47pF (showed Figure 6).

ENVIRONMENTAL CONDITIONS INFLUENCE

In this part a lot of papers have been performed on ceramic capacitors X7R type. We have synthesized different results in different European programs: AMELIE (FP6) and PROCURE (FP7) and some results coming from various papers.

TEMPERATURE IMPACT ON CAPACITANCE

After the reflow, the ceramic capacitors were stored at high temperature 150 °C and 85°C.

To measure the impact of the storage we used impedance spectroscopy based on piezoelectricity analysis existing on such capacitors [6] (example done Figure 7). After 1000h a strong dispersion of the impedance is observed. Two devices are defective (cracks). The position of weak devices is on the left zone of the Gaussian curve at 1000h (see arrows on Figure 8). At t=0h the impedance value of both components was external values of the Gaussian. These quality problems have been resolved by the manufacturer in the European program PROCURE -FP5 [6]. The decoupling capacitors (X7R) have become more robust by increasing the nickel barrier and by adding dopants.

MOISTURE IMPACT ON CAPACITANCE

Usually the humidity is not a recognized factor of failure for ceramic capacitors. DONAHOE Daniel N. and all [START_REF] Daniel | Sanka Ganesan -Moisture induced degradationof multilayer ceramic capacitors[END_REF] in their paper intituled "Moisture induced degradation of multilayer ceramic capacitors" explained that, when both precious metal electrode and base metal electrode (BME) capacitors were subjected to autoclave (121 °C/ 100% RH) testing after 500h, it was found that the precious metal capacitors aged according to a well known aging mechanism (average capacitance degraded less than 3% from their starting values), but the BME capacitors degraded to below the -30% specification limit.

DISCUSSION

As shown in Figure 9, the expected values of capacitance for the next years will be around 1000 μF/mm 2 . These values come from the compilation of data from different manufacturers of ceramic capacitors. These values are smoothed, showing the tendency to be limited by the grain size of BaTiO 3 .

The decrease of the thickness of the dielectric is influenced by the use of nanoscale grains. We note the risk introduced by a thin layer on the stiffness of these layers. The strain value on the top and the bottom of the dielectric inside the PCB were calculated using an analytical program (CRITIDEL from EURELNET). The obtained values are less than the values encountered generally for a capacitor reflowed on a PCB [START_REF] Chan 1 | Failure analysis of miniaturized multilayer ceramic capacitors in surface mount printed circuit board assemblies[END_REF]: close to 8 MPa to 10 MPa instead of 200 MPa to 300 MPa for the reflow process.

For electronic designers embedded capacitors are promising but the range value is not sufficient for the moment. They need a higher value of dielectric permittivity to reduce the size of the elementary capacitor and generate in one shot different kind of capacitance values. Today these values are closed to 0.5 to 2 nF/cm². The best will be 20 to 200 nF/cm², for a larger capacitance value range.

CONCLUSION

Whatever the studied organic polymer dielectric, the capacitance decreases after thermal exposure (max ≈ 6%) because of water evaporation.

We can note a slope change after thermal exposure: there is a possible dielectric properties modification due to temperature (ε' increase after thermal ageing).

Whatever the studied organic polymer dielectric, the capacitance increases after humidity exposure (max ≈ 6%) due to water absorption.

Embedded passives are more reliable by eliminating solder joints. Strains at the different interfaces are less than a solder joint good result for the reliability point of view.

With the added benefits of elimination of solder joints, inventory, assembly and rework vis-a-vis their surface mounted counterparts, embedded ceramic capacitors provide a very convincing and compelling option to meet the ever increasing need for miniaturization and improve performance in modern electronic designs.

Figure 1 :

 1 Figure 1: EDX analysis of a commercial dielectric layer based to BaTiO 3 .

Figure 2 :

 2 Figure 2: Capacitance variation Vs frequency in the temperature range ([25-150°C].

Figure 3

 3 Figure3shows the capacitance variation versus frequency after various exposure times to (a) 85°C and (b) 150°C for the first exposure hours.

Figure 3 :

 3 Figure 3: Capacitance variation Vs frequency after various exposure times to (a) 85°C and (b) 150°C

Figure 4 :

 4 Figure 4: Capacitance variation (1*1 mm²) Vs frequency after various exposure times to 150°C

Figure 6 :

 6 Figure 6: PCB test vehicle for ceramic capacitors in AMELIE FP6 program.

  The piezoelectric test was performed just before the ageing test (storage) and at 200hours, 600hours and 1000hours. At 1000hours two components present a weakened piezoelectric signature. The parameters are: Number of cracked devices after 600h: 0 Number of cracked devices after 1000h: 2 Number of devices close of the failure (crack) after 1000h: 2 Number of devices with Ri failure: 0, Ri > 1011 Ohm Drift of the capacitance observed at the end of ageing.

Figure 7 :

 7 Figure 7: Example of piezoelectric response for a defective capacitor.

Figure 8 :

 8 Figure 8: Statically dispersion of the impedance during ageing test.

Figure 9 :Figure 10 :

 910 Figure 9: Evolution of the main parameters from different manufacturers of ceramic capacitors

ACKNOWLEDGEMENTS

The authors gratefully acknowledge different manufacturers which provide us materials for the tests within program research and development such as HIFAP (PIDEA+), PROCURE FP5 and AMELIE FP6.