5.10.11. 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 desirable to avoid fragmentation, although it is less of a problem in a system with a good buffer cache with read-ahead.

Modern Linux filesystem keep fragmentation at a minimum by keeping all blocks in a file close together, even if they can't be stored in consecutive sectors. Some filesystems, like ext3, effectively allocate the free block that is nearest to other blocks in a file. Therefore it is not necessary to worry about fragmentation in a Linux system.

In the earlier days of the ext2 filesystem, there was a concern over file fragmentation that lead to the development of a defragmentation program called, defrag. A copy of it can still be downloaded at http://www.go.dlr.de/linux/src/defrag-0.73.tar.gz. However, it is HIGHLY recommended that you NOT use it. It was designed for and older version of ext2, and has not bee updated since 1998! I only mention it here for references purposes.

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 defragmenting is a good idea for all filesystems, since many things can go wrong during the defragmentation.