Filesystem images in local files can be used by many PC emulators and virtual machines (user-mode Linux, QEMU and Xen, to name but three). Typically these filesystems are created as sparse files using commands like:
dd if=/dev/zero of=fs.image bs=1024 seek=2000000 count=0 /sbin/mke2fs fs.imagewhere the enormous
ddto move forward by 2GB before writing nothing at all. This results in the creation of a sparse file which takes disk space only for blocks which are actually used:
$ ls -l fs.image -rw-rw-r-- 1 rmy rmy 2048001024 Apr 18 19:10 fs.image $ du -s fs.image 31692 fs.imageAs the filesystem is used, more and more of the non-existent blocks are filled with data and the size of the file on disk grows. Sometimes it would be nice to be able to reclaim unused blocks from a filesystem image. However, deleting files from the image doesn't return the space to the underlying filesystem: even free blocks in the image still consume space. Reclaiming the space can be achieved in two stages:
One traditional way to zero unused blocks is to create a file that fills all the free space:
dd if=/dev/zero of=junk sync rm junk
The disadvantage of
dd in this context is that it destroys
any sparseness that exists: free blocks that were originally represented
as holes in the image file are replaced with actual blocks containing
zeroes. Also, filling up a live filesystem is probably a bad idea.
As an alternative approach, and as practice in mucking about with ext2 filesystems, I've written a utility which scans the free blocks in an ext2 filesystem and fills any non-zero blocks with zeroes. The source, zerofree-1.1.1.tgz, is available for download.
However, this is only half the story: the empty free blocks still consume space in the underlying filesystem, so something must to be done to reclaim that space.
A common suggestion is to use the sparse file handling capabilities
of the GNU
cp command to take a copy of the filesystem image with
cp --sparse=always (though this does require the original
and sparse files to exist at the same time, which may be inconvenient).
If your kernel and util-linux are sufficiently modern and you have a supported
filesystem you can use
fallocate -d to 'dig holes' in a file.
This makes the file sparse in-place, without using extra disk space.