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Re: FYI: LSI rebuilding; and XFS speed V. raw - hints on maxing out 'dd

To: Emmanuel Florac <eflorac@xxxxxxxxxxxxxx>
Subject: Re: FYI: LSI rebuilding; and XFS speed V. raw - hints on maxing out 'dd'....(if not already obvious)...
From: Linda Walsh <xfs@xxxxxxxxx>
Date: Sat, 11 Jun 2011 09:48:49 -0700
Cc: Paul Anderson <pha@xxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20110611113049.2868f9c7@xxxxxxxxxxxxxxx>
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Emmanuel Florac wrote:
Le Fri, 10 Jun 2011 18:33:08 -0700 vous écriviez:

I have a 9285-8E and have been pretty happy with it's performance,
but I only have 10 data disks (2x6-disk RAID5 =>RAID50) with 2TB
SATA's and get 1GB perf... about what I'd expect from disks that get
around 120MB each and doing 2 RAID5 calcs...)...

I've made some more tests since then, and I was using cheapo drives for
testing that for some reason, behave extraordinarily poorly in
combination with the LSI card. I've retested the card with hitachi
Ultrastars and it worked just fine (though a bit slower than Adaptec
6xx5). OTOH, the Adaptec works fine with the "bad" drives too, go
figure.
-----
        I noticed this issue with the LSI card as well -- i.e. with
'bad' low-qual drives.

What I noticed though -- in independent testing of the lower quality drives, was that even though they were all spec'ed at
7200RPM, their actual speeds varied, from top to bottom by almost 50%
(Indirectly measured by top transfer speeds copying from & to the drives
128G files... .. all freshly formatted, factory 'new', from the same
generation).   Speeds varied from top to bottom with max around 129MB/s
to bottom @ 87MB/s, with typical being 95-120. That was quite a range of speeds. Trying to 'synchronize reads/writes/ on drives with such varying speeds would be a nightmare.
        I tried another RAID controller and got about 60% the performance
of the LSI's -- but it was also with the cheaper drives.  The LSI controller
wouldn't "pass" the drives as "acceptable" if they were too far out of spec.

        The Ultrastars (NOT the deskstars, which are not speed controlled)
all were fine.  I've heard of people having problems with Enterprise
Class WD drives due to the issue.   So it may be a matter of the controller
'protecting itself' from drives that are out of spec -- and are therefore not
considered 'RAID class' drives...
        Some would argue that being more 'tolerant' of poor drives is a good
thing...  But if you want the fastest speed, you need to make sure the drive
speeds are well matched.




same issue.

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