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Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)

To: Chris Snook <csnook@xxxxxxxxxx>
Subject: Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)
From: Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>
Date: Wed, 28 May 2008 15:27:58 -0400 (EDT)
Cc: linux-kernel@xxxxxxxxxxxxxxx, linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <483DB0EC.3090403@redhat.com>
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On Wed, 28 May 2008, Chris Snook wrote:

Justin Piszcz wrote:

On Wed, 28 May 2008, Chris Snook wrote:

Justin Piszcz wrote:

Given that one of the greatest benefits of NCQ/TCQ is with parity RAID, I'd be fascinated to see how enabling NCQ changes your results. Of course, you'd want to use a single SATA controller with a known good NCQ implementation, and hard drives known to not do stupid things like disable readahead when NCQ is enabled.
Only/usually on multi-threaded jobs/tasks, yes?

Generally, yes, but there's caching and readahead at various layers in software that can expose the benefit on certain single-threaded workloads as well.

Also, I turn off NCQ on all of my hosts that has it enabled by default because
there are many bugs that occur when NCQ is on, they are working on it in the
libata layer but IMO it is not safe at all for running SATA disks w/NCQ as
with it on I have seen drives drop out of the array (with it off, no problems).

Are you using SATA drives with RAID-optimized firmware? Most SATA manufacturers have variants of their drives for a few dollars more that have firmware that provides bounded latency for error recovery operations, for precisely this reason.
I see--however, as I understood it there were bugs utilizing NCQ in libata?

But FYI--
In this test, they were regular SATA drives, not special raid-ones (RE2,etc).

Thanks for the info!


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