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

To: Ragnar Kjørstad <xfs@xxxxxxxxxxxxxxxxxxx>
Subject: Re: config question (external raid, external log?)
From: Simon Matter <simon.matter@xxxxxxxxxxxxxxxx>
Date: Tue, 28 May 2002 08:19:04 +0200
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Cc: Robert Sander <gurubert-news@xxxxxxxxxxxxxxx>, linux-xfs@xxxxxxxxxxx
Organization: Sauter AG, Basel
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Ragnar Kjørstad schrieb:
> 
> 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.

I agree when we're only talking about hardware RAID, although I don't
think having external logs on the same spindles hurts performance much.

With software RAID and XFS it is completely different. At least with
RAID5 it's a must to have external logs and it doesn't hurt if it's on a
separate RAID1 on the same spindles. I've been running lots of mixed
load tests on two servers with IDE and SCSI/software and hardware RAID
last year to find out how I have to setup different configurations.

Simon

> 
> 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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