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

[Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>]

On Thu, 29 May 2008, Kasper Sandberg wrote:

> 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?
Not off-hand, check LKML and my email address from early this year or last 
year and/or the ide-list.

Justin.

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