Fighting fragmentation

When a file is written to disk, it can't always be written in consecutive blocks. A file that is not stored in consecutive blocks is fragmented. It takes longer to read a fragmented file, since the disk's read-write head will have to move more. It is desireable to avoid fragmentation, although it is less of a problem in a system with a good buffer cache with read-ahead.

The ext2 filesystem attempts to keep fragmentation at a minimum, by keeping all blocks in a file close together, even if they can't be stored in consecutive sectors. Ext2 effectively always allocates the free block that is nearest to other blocks in a file. For ext2, it is therefore seldom necessary to worry about fragmentation. There is a program for defragmenting an ext2 filesystem, see [TV] in the bibliography.

There are many MS-DOS defragmentation programs that move blocks around in the filesystem to remove fragmentation. For other filesystems, defragmentation must be done by backing up the filesystem, re-creating it, and restoring the files from backups. Backing up a filesystem before defragmening is a good idea for all filesystems, since many things can go wrong during the defragmentation.