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Unformatted text preview: Improving Data Consistency in Mobile Computing Using Isolation-Only Transactions Qi Lu M. Satyanarayanan School of Computer Science Carnegie Mellon University Pittsburgh, PA 15213 luqi,satya @cs.cmu.edu Abstract Disconnected operation is an important technique for providing mobile access to shared data in distributed file systems. However, data inconsistency resulting from par- titioned sharing remains a serious concern. This paper presents a new mechanism called isolation-only transac- tion(IOT) that uses serializability constraints to automat- ically detect read/write conflicts. The IOT consistency model provides a set of options for automatic and manual conflict resolution. In addition, application specific knowl- edge can be incorporated to detect and resolve conflicts. To preserve upward Unix compatibility, the IOT mecha- nism is provided as an optional file system facility and its flexible interfaces allow any existing Unix application to be executed as an IOT. This paper describes high level sys- tem design and implementationand concludes with related work and current status. 1 Introduction Disconnected operation based on an optimistic replica control strategy has proved to be a viable technique for mobile file access. There is now considerable evidence that write/write conflicts, which dominated early discus- sions of optimistic replication, are relatively rare and can often be transparently resolved[10, 11, 12, 18]. But actual experience indicates that read/write conflicts, traditionally ignored in file systems, deserve much greater attention in disconnected operation. The possibility of read/write conflicts exists even in timesharing file systems: for example, consider the in- stallation of a new version of a library by one user while another user is building executables that link in the library. Although such conflicts are rare in timesharing or worksta- tion environments, the analogous scenario in mobile com- puting is far more likely to lead to read/write conflicts. Two factors account for this. First, long periods of discon- nection significantly enlarge the window of vulnerability. This research was supported by the Air Force Materiel Command (AFMC) and ARPA under contract number F196828-93-C-0193. Addi- tional support was provided by the IBM Corporation, Digital Equipment Corporation, and Intel Corporation. Second, explicituser coordinationtoavoid conflictsis more difficult. Consider the following example of a partitioned read/write conflict. A programmer Joe caches relevant files on his laptop for a weekend trip. While disconnected, he edits some source files and builds a new version of cfs , a file system utility program. But one of the libraries libutil.a that is linked in is updated on the servers during Joes absence. Here the linking and updating of libutil.a constitute a read/write conflict, leaving cfs in a possible unsatisfactory state. The consequences of un- detected read/write conflicts can be especially painful if the...
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- Fall '09