A Holesome File System
[vengroff:holesome-tr]Darren Erik Vengroff and David Kotz. A Holesome File System. Technical Report number TR2004-497, Dartmouth Computer Science, May 2004. ©Copyright the authors. Originally written in July 1995; released May 2004.
We present a novel approach to fully dynamic management of physical disk blocks in Unix file systems. By adding a single system call, zero(), to an existing file system, we permit applications to create holes, that is, regions of files to which no physical disk blocks are allocated, far more flexibly than previously possible. zero can create holes in the middle of existing files.
Using zero(), it is possible to efficiently implement applications including a variety of databases and I/O-efficient computation systems on top of the Unix file system. zero() can also be used to implement an efficient file-system-based paging mechanism. In some I/O-efficient computations, the availability of zero() effectively doubles disk capacity by allowing blocks of temporary files to be reallocated to new files as they are read.
Experiments on a Linux ext2 file system augmented by zero() demonstrate that where their functionality overlaps, zero() is more efficient than ftruncate(). Additional experiments reveal that in exchange for added effective disk capacity, I/O-efficient code pays only a small performance penalty.
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