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<H2><A NAME="SECTION00588000000000000000">Fighting fragmentation</A></H2>
<P>
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 <b>fragmented</b>. 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.
<P>
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 [<A HREF="node113.html#ext2-defrag">TV</A>] in the bibliography.
<P>
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.
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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