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Coda (Constant Data Availability) is a distributed file system developed as a research project at Carnegie Mellon University since 1987 under the direction of Mahadev Satyanarayanan. It descended directly from an older version of Andrew File System (AFS-2) and offers many similar features. The InterMezzo file system was inspired by Coda. Coda is still under development, but there has not been any activities on their website since February 2011.〔(【引用サイトリンク】title=Coda File System: Ticket #1680 (new defect): Venus mount problems )〕 Though there are claims that the focus shifted from research to creating a robust product for commercial use, but activities about the system in linux system kernel is rather low.〔(【引用サイトリンク】title=Linux-2.6.12-rc2 )〕 ==Features== Coda has many features that are desirable for network file systems, and several features not found elsewhere. # Disconnected operation for mobile computing. # Is freely available under the GPL〔(【引用サイトリンク】title=New release: 5.0.pre1 )〕 # High performance through client side persistent caching # Server replication # Security model for authentication, encryption and access control # Continued operation during partial network failures in server network # Network bandwidth adaptation # Good scalability # Well defined semantics of sharing, even in the presence of network failures Coda uses a local cache to provide access to server data when the network connection is lost. During normal operation, a user reads and writes to the file system normally, while the client fetches, or "hoards", all of the data the user has listed as important in the event of network disconnection. If the network connection is lost, the Coda client's local cache serves data from this cache and logs all updates. This operating state is called disconnected operation. Upon network reconnection, the client moves to reintegration state; it sends logged updates to the servers. Then it transitions back to normal connected-mode operation. Also different from AFS is Coda's data replication method. AFS uses a pessimistic replication strategy with its files, only allowing one read/write server to receive updates and all other servers acting as read-only replicas. Coda allows all servers to receive updates, allowing for a greater availability of server data in the event of network partitions, a case which AFS cannot handle. These unique features introduce the possibility of semantically diverging copies of the same files or directories, known as "conflicts". Disconnected operation's local updates can potentially clash with other connected users' updates on the same objects, preventing reintegration. Optimistic replication can potentially cause concurrent updates to different servers on the same object, preventing replication. The former case is called a "local/global" conflict, and the latter case a "server/server" conflict. Coda has extensive repair tools, both manual and automated, to handle and repair both types of conflicts. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Coda (file system)」の詳細全文を読む スポンサード リンク
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