Disk defragmenter in Linux

[Mike McCarty]

Tony Nelson wrote:
> At 2:32 PM +0700 12/23/05, Fajar Priyanto wrote:
> 
>>Hi all,
>>A today's mail from one of the new comer brings me this question again in my
>>mind. Linux disk defragmenter. Does it really not needed?
>>
>>I've been googling around and find that this matter has been discussed as
>>early as 1998. And it seems that the only distro that provides a defragmenter
>>program is debian.
>>
>>There are several way of fixing a heavy defragmented disk in Linux, but the
>>easiest way is to copy all of the content of the partition into another
>>place, completely erase that partition, and copy back the content.
>>
>>My own experience shows me just that. My /home partition was almost full with
>>only 2% freespace. During that time, my Kmail became very slow such as when
>>downloading email or when I moved between mail folders. The harddisk was just
>>spinning all the time.
>>
>>Then I copy all my files and mails from the /home partition and move them all
>>to another partition. Then delete them from /home. After that, I copied some
>>of the files and mail back to /home in order to keep 20% of /home free. So
>>far the performance is ok.
>>
>>However, still the question remains. If Linux ext3 doesn't need defragmenter,
>>and able to defrag itself, what is the process name? And when does it run?
>>Can I see it in action? Is there an utility to see on what percentage my
>>current defragmentation? I tried fschk but no luck.
> 
> 
> The opinion that EXT2 doesn't need defragmenting is based on only a
> filesystem-level view of the problem, and doesn't consider data read and
> write performance.  EXT2 does make an effort to keep data only a short seek
> away ("clustered").  With this clustering, the filesystem operations of
> adding, removing, extending, and shortening files are not much affected by
> fragmentation.
> 
> With EXT3 (journalling), which always writes data to a new place, updates
> the filesystem info, and then frees the old data (roughly speaking),
> fragmentation is a way of life, and there isn't much to be done about it.
> Clustering helps by keeping the seeks relatively short, if there is space
> nearby.

I've heard this argument before, quite a few times in fact. It ignores
one big fact of life with regards to discs. Almost all the data on
my disc are *static*. They don't change. And so having files (like
/bin/ls, for example) be contiguous saves enormously when they are
read. Even if most of the data on one's disc is not static, still
quite a bit of it is, or should be. (I'll omit to discuss the
abominable prelink for the moment.)

> When you have only 2% free, it's just about certain that the free space is
> a long way away from the rest of the data in a file.  Just deleting to get
> 20% free would probably have fixed your problem.

Absolutely. Running with just that much free on a Linux system is
insanity, anyway. I'd be afraid of a system lockup.

Mike
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