[ALUG] Defragmenting Hard drive

[Wayne Stallwood]

On Tue, 2007-07-31 at 18:24 +0100, Laurie Brown wrote:

> WHAT??!!!!!!!!!
> 
> Sorry, massive disagreement from here... NTFS fragments dreadfully, take 
> a look after a defrag, then say, install MS Office, then take another look.
> 

Warning..long answer ahead

NTFS does still suffer from fragmentation problems, not as badly as FAT
as the filesystem logic is a bit cleverer about block allocation. Part
of the illusion of NTFS coping better is that modern disks cope
better..but there is an improvement over FAT for sure

ext2/3 also suffer (it is the non contiguous value you get when you run
fsck) however the problem is far less rampant and usually only happens
on really old (and heavily modified when full) partitions. There are no
stable tools I know of to fix fragmentation on ext2/3 and given how
small the problem is there are unlikely to ever be any. ext suffers a
lot less than FAT as it pre-allocates blocks beyond a given file length
to allow for future growth. This however does not deal with the
fragmentation problems you get on a nearly full filesystem which is why
for optimum performance it is good to run ext2/3 with some free space.

If you do get into a problem with fragmentation (which is highly
unlikely as generally the non contiguous value stays below say 5%) then
the only completely safe fix I am aware of is to copy the data off and
reformat.

Even in the cases of heavy fragmentation on Linux the performance impact
is almost negligible compared to the days of MSDOS due to the way that
modern kernels access information on a drive. MSDOS and old versions of
Windows assumed single user, single process, single file, disk access.
and so only retrieved what it needed from the disk. Linux on the other
hand retrieves pretty much whatever passes under the drive head and it
can hold in cache (using spare memory) whether it thinks it needs it or
not..given that most file fragmentation happens over a small block of
clusters there is a very reasonable chance that the rest of the
fragmented file is already in cache as the heads made their way to what
was requested.   

Most if not all of this is also done in NTFS and modern NT kernels, so
although NTFS does become fragmented (and generally more so than Linux
filesystems) the actual performance benefit of correcting it is not as
obvious as in the days of FAT