FAST '03 Abstract
Metadata Efficiency in Versioning File Systems
Craig A. N. Soules, Garth R. Goodson, John D. Strunk, and Gregory R. Ganger, Carnegie Mellon University
Versioning file systems retain earlier versions of modified
files, allowing recovery from user mistakes or system
corruption. Unfortunately, conventional versioning
systems do not efficiently record large numbers of versions.
In particular, versioned metadata can consume
as much space as versioned data. This paper examines
two space-efficient metadata structures for versioning
file systems and describes their integration into the Comprehensive
Versioning File System (CVFS), which keeps
all versions of all files. Journal-based metadata encodes
each metadata version into a single journal entry; CVFS
uses this structure for inodes and indirect blocks, reducing
the associated space requirements by 80%. Multiversion
b-trees extend each entry’s key with a timestamp and
keep current and historical entries in a single tree; CVFS
uses this structure for directories, reducing the associated
space requirements by 99%. Similar space reductions
are predicted via trace analysis for other versioning
strategies (e.g., on-close versioning). Experiments with
CVFS verify that its current-version performance is sim-ilar
to that of non-versioning file systems while reducing
overall space needed for history data by a factor of two.
Although access to historical versions is slower than con-ventional
versioning systems, checkpointing is shown to
mitigate and bound this effect.
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