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

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Date:	Thu, 29 May 2008 11:57:30 +0200
From:	Kasper Sandberg <lkml@...anurb.dk>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
Cc:	Chris Snook <csnook@...hat.com>, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, xfs@....sgi.com
Subject: Re: Performance Characteristics of All Linux RAIDs (mdadm/bonnie++)

On Wed, 2008-05-28 at 15:27 -0400, Justin Piszcz wrote:
> 
> On Wed, 28 May 2008, Chris Snook wrote:
> 
> > Justin Piszcz wrote:
> >> 
> >> 
> >> On Wed, 28 May 2008, Chris Snook wrote:
> >> 
> >>> Justin Piszcz wrote:
> >>>> Hardware:
> >>>> 
> >>> 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?
You wouldnt happen to have some more information about this? i havent
personally had problems yet, but i havent used it for very long - but
since it comes activated by DEFAULT, i would assume it to be relatively
stable?

> 
> But FYI--
> In this test, they were regular SATA drives, not special raid-ones (RE2,etc).
> 
> Thanks for the info!
> 
> Justin.
> 
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