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Re: config question (external raid, external log?)

To: Robert Sander <gurubert-news@xxxxxxxxxxxxxxx>
Subject: Re: config question (external raid, external log?)
From: Ragnar Kjørstad <xfs@xxxxxxxxxxxxxxxxxxx>
Date: Mon, 27 May 2002 21:36:53 +0200
Cc: linux-xfs@xxxxxxxxxxx
In-reply-to: <news2mail-slrnaf3t7i.c3t.gurubert-news@xxxxxxxxxxxxxxxxxxxxx>; from ml-linux-xfs@xxxxxxxxxxxxxxx on Mon, May 27, 2002 at 08:57:23AM +0000
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On Mon, May 27, 2002 at 08:57:23AM +0000, Robert Sander wrote:
> We will have a new RAID-System shipped the next weeks with a total
> cpacity of 1.6 TB. I want to run XFS (what else?) on it and have some
> config qeustions.
> The RAID consists of 12 160GB IDE-Disks connected via SCSI to the host
> computer. The host only sees one large SCSI disk.
> I think that making just one partition on that disk and running a
> mkfs.xfs without any special options will not produce optimal
> performance.
> Should I create a separate logfile partition on the RAID? How large
> would it be? What are other options for mkfs.xfs that I should look
> into?

Putting the logfile on a seperate partition on the same raid-array is
likely to give worse performance. (because it will cause longer seeks)

The only way a seperate log-device can be benefitial is if it's on a
seperate raid-array so it uses different spindels than the
data-partition. E.g. you could set aside 2 disks for a raid1 array for
the log, but this will 
a) waste a lot of space
b) the log-partition will have lower throughput (as it has fewer
   spindels) so you may end up with a slower system after all.

It depends on your application, but in general I would say that external
logs only make sense for extreeme performance needs, and then you should
use a high-performance redundant storage (e.g. mirrored NVRAM).

Ragnar Kjorstad
Big Storage

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