Disk defragmenter in Linux

[John Summerfied]

Tim wrote:
> Tim:
> 
>>>But such (static) data doesn't get fragmented, it stays as it was
>>>original written.  It's changing files that become fragmented, and
>>>newly created ones
> 
> 
> 
> Mike McCarty:
> 
>>Er? Perhaps what Tony wrote was in error, but his understanding is
>>the same as mine. The ext3 tends to fragment files as it writes them.
> 
> 
> It would only be fragmenting the files that it writes to, not the ones
> already on the disk.  Sure, a fragmented word processor document might
> take a bit longer to open (though it'd have to be a large file for you
> to notice), but the word processor is going to take just as long to
> start up as it ever did.  Likewise with all the other unmodified files
> on the drive (most of the OS and applications).  Writing a fragmented
> file doesn't shuffle everything else around.
> 
> Things like large mail spool files have been about the only thing that
> strike me as a fragmentation issue.  Most other files are rather small.
> 
> 
>>And what you wrote doesn't address the directories, which get appended
>>to, and presumably fragmented, at the time they are creat
> 
> 
> I was under the impression that the directory structure was recorded in
> manner that's different from how the files are stored.
> 


Might I observe that the many-partitions layout so often recommended 
gives you all the disadvantages of a fragmented drive from day one?

Two busy partitions is one too many. In these times of cheap disks and 
USB2 enclosures, I'd rather have one partition for everything (except 
maybe /boot and maybe other low-use stuff), and if an upgrade is 
contemplated, back up /home to a USB drive. At worst, almost anything 
can be backed up overnight. According to dd, I can backup /dev/hda (80 
Gb) to a USB2 disk at 14 Mbytes/sec on my laptop.

Arguably, I should be doing something of the sort regardless. As should 
anyone with many computers in their care.

fwiw I used to use OS/2 and it was IBM's recommendation that one should 
not defrag HPFS (which, btw, predates NTFS) partitions because HPFS 
allocates space in bands (take your pad, divide it into eight columns 
and you have eight bands) (and so takes an initial performance hit). 
File expansions are done within the same band where possible, so 
reducing the impact of further fragmentation. Performance was pretty 
uniform up to, I think, about 95% full.

Defragging an HPFS drive would involve putting all the files together 
into a single block, and the chances were good that you'd soon find 
files occupying extents both inside and outside that block and 
consequent bad performance.

I've always assumed that, since the algorithms have existed for a long 
time, that Linux filesystems are also good in that respect. The fact 
that no defragger is included in populare distros supports my 
(underinformed) view.

Journalling introduces a complication, but its effect depends on where 
the journal is. Also, journalling only has an effect when files are written.

Finally, the ultimate defag tool is backup and restore. It might not be 
necessary, but it won't do any harm either.




-- 

Cheers
John

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