Re: defrag on linux system

From: Michel Bardiaux (michel.bardiaux_at_mediaxim.be)
Date: 08/31/05 

Date: Wed, 31 Aug 2005 14:48:58 +0200

Tuukka Toivonen wrote:
> On 2005-08-24, Peter T. Breuer <ptb@oboe.it.uc3m.es> wrote:
>
>>>"The more fragmented the volume is, the slower the computer's file input
>>>and output performance will be."
>>
>>That's obviously false in general - I described why. It's only true on
>>a system on which ONE process is running, which is not the modern case.
>
>
> [....]
>
> Not exactly on the topic, but fragmentation is a huge problem to me.
> I have striping RAID 2*120 GB and would like to capture real time
> uncompressed video on it, 768x576x25x1.5 = 15.8 MB/sec.
>
> Measuring with hdparm, the disks could do 40-50 MB/sec, but
> with reiserfs, it's just doesn't work, too slow.
>
> Using ext2 with _unfragmented_ filesystem, it works, but
> I would need to keep the file system practically always
> empty or it gets too fragmented. And this is very difficult
> since I don't have other so large disks.
>
> Does anyone know how could I defragment the disks,
> in particular free space?
> "defrag" seems to defragment _allocated_ space
> (optimizing read speed), but apparently not the empty
> space (_not_ optimizing write speed).
>

I have similar problems with disks holding realtime compressed video.
Old files are perpetually destroyed to make room for new ones while
keeping the disk 99% full, and that seems to be one of those rare cases
where fragmentation *does* grow with time. Unfortunately the idea that
fragmentation could matter even in special cases is heresy to many, and
its almost impossible to have a serene discussion about it. Nevertheless
one useful suggestion was made: preallocate the files, never destroy
them but recycle by renaming.

In your case with raw video the size is *exactly* predictable so that
strategy should work even better.

Greetings, 

-- 
Michel Bardiaux
R&D Director
T +32 [0] 2 790 29 41
F +32 [0] 2 790 29 02
E mailto:mbardiaux@mediaxim.be
Mediaxim NV/SA
Vorstlaan 191 Boulevard du Souverain
Brussel 1160 Bruxelles
http://www.mediaxim.com/

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