From: Dave Chinner <david@fromorbit.com>
To: Stan Hoeppner <stan@hardwarefreak.com>
Cc: xfs@oss.sgi.com
Subject: Re: Verify filesystem is aligned to stripes
Date: Thu, 25 Nov 2010 21:15:37 +1100 [thread overview]
Message-ID: <20101125101537.GD12187@dastard> (raw)
In-Reply-To: <4CEE0995.9030900@hardwarefreak.com>
On Thu, Nov 25, 2010 at 01:00:37AM -0600, Stan Hoeppner wrote:
> Dave Chinner put forth on 11/24/2010 11:46 PM:
>
> > Because writes for workloads like this are never full stripe writes.
> > Hence reads must be done to pullin the rest of the stripe before the
> > new parity can be calculated. This RMW cycle for small IOs has
> > always been the pain point for stripe based parity protection. If
> > you are doing lots of small IOs, RAID1 is your friend.
>
> Do you really mean RAID1 here Dave, or RAID10? If RAID1, please
> elaborate a bit.
RAID10 is just a convenient way of saying "striped mirrors" or
"mirrored stripes". Fundamentally they are still using RAID1 for
redundancy - a mirror of two devices. A device could be a single
drive or a stripe of drives.
> RAID1 traditionally has equal read performance to a
> single device, and half the write performance of a single device.
A good RAID1 implementation typically has the read performance of
two devices (i.e. it can read from both legs simultaneously) and the
write performance of a single device.
Parity based RAID is only fast for large write IOs or small IOs that
are close enough together that a stripe cache can coalesce them into
large writes. If this can't be acheived, parity based raid will be
no faster than a _single drive_ for writes because all drives will
be involved in RMW cycles. Indeed, I've seen RAID5 luns be saturated
at only 50 iops because every IO required a RMW cycle, while an
equivalent number of drives using RAID1 of RAID0 stripes did 1,000
iops...
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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next prev parent reply other threads:[~2010-11-25 10:14 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-11-24 18:39 Verify filesystem is aligned to stripes Spelic
2010-11-25 5:46 ` Dave Chinner
2010-11-25 7:00 ` Stan Hoeppner
2010-11-25 10:15 ` Dave Chinner [this message]
2010-11-25 22:57 ` Stan Hoeppner
2010-11-26 8:16 ` Emmanuel Florac
2010-11-26 12:22 ` Dave Chinner
2010-11-26 13:15 ` Spelic
2010-11-26 14:05 ` Michael Monnerie
2010-11-26 14:36 ` Emmanuel Florac
-- strict thread matches above, loose matches on Subject: below --
2010-11-26 2:43 Richard Scobie
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