Modern file systems associate the deletion of a file with the immediate release of storage, and file writes with the irrevocable change of file contents. We argue that this behavior is a relic of the past, when disk storage was a scarce resource. Today, large cheap disks make it possible for the file system to protect valuable data from accidental delete or overwrite. This paper describes the design, implementation, and performance of the Elephant file system, which automatically retains all important versions of user files. Users name previous file versions by combining a traditional pathname with a time when the desired version of a file or directory existed. Storage in Elephant is managed by the system using filegrain user-specified retention policies. This approach contrasts with checkpointing file systems such as Plan-9, AFS, and WAFL that periodically generate efficient checkpoints of entire file systems and thus restrict retention to be guided by a single policy for all files within that file system. Elephant is implemented as a new Virtual File System in the FreeBSD kernel.

Modern file systems associate the deletion of a file with the release of the storage associated with that file, and file writes with the irrevocable change of file contents. We propose that this model of file system behavior is a relic of the past, when disk storage was a scarce resource. We believe that the correct model should ensure that all user actions are revocable. Deleting a file should change only the name space and file writes should overwrite no old data. The file system, not the user, should control storage allocation using a combination of user specified policies and information gleaned from file-edit histories to determine which old versions of a file to retain and for how long. This paper presents the Elephant file system, which provides users with a new contract: Elephant will automatically retain all important versions of the users files. Users name previous file versions by combining a traditionalpathname with a time when the desired version of a file or directory existed. Elephant manages storage at the granularity of a file or groups of files using user-specified retention policies. This approach contrasts with checkpointing file systems such as Plan-9, AFS, and WAFL, that periodically generate efficient checkpoints of entire file systems and thus restrict retention to be guided by a single policy for all files within that file system. We also report on the Elephant prototype, which is implemented as a new Virtual File System in the FreeBSD kernel. 1.

The ext3cow file system, built on Linux's popular ext3 file system, brings snapshot functionality and file versioning to the open-source community. Our implementation of ext3cow has several desirable properties: ext3cow is implemented entirely in the file system and, therefore, does not modify kernel interfaces or change the operation of other file systems; ext3cow provides a time-shifting interface that permits access to data in the past without polluting the file system namespace; and, ext3cow creates versions of files on disk without copying data in memory. Experimental results show that the time-shifting functions of ext3cow do not degrade file system performance. Ext3cow performs comparably to ext3 on many file system benchmarks and trace driven experiments.

This paper describes the design and implementation of Mammoth, a novel distributed file system that we are building. The key feature of Mammoth is that it stores files and directories as histories of immutable versions that are created each time a file or directory is modified. All Mammoth meta-data is stored in append-only fashion and all file data is read-only once it is created. As a result, Mammoth can replicate data on multiple loosely-connected nodes with minimal concern for consistency. The only overhead for creating and maintaining a replica is the storage it occupies.

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