Document Type

Technical Report

Publication Date


Technical Report Number



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.


This paper was written in July 1995, but sat unpublished until May 2004 when it was released as this technical report, unchanged.