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Re: [[PATCH, RESEND]] less AGs for single disks configs.

To: Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>
Subject: Re: [[PATCH, RESEND]] less AGs for single disks configs.
From: David Chinner <dgc@xxxxxxx>
Date: Tue, 13 Nov 2007 07:31:47 +1100
Cc: xaiki@xxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <Pine.LNX.4.64.0711120956340.2701@p34.internal.lan>
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On Mon, Nov 12, 2007 at 09:57:21AM -0500, Justin Piszcz wrote:
> On Mon, 12 Nov 2007, David Chinner wrote:
> >A single spindle, regardless of it's size, will have similar
> >seek characteristics so scaling the number of AGs with size
> >is the wrong thing to do - you don't get better parallelism
> >out of a single spindle, just more seeks and lower performance.
> >hence keeping the number of AGs fixed up to the point where
> >the AG size tops out (i.e. 4TB) seems like a better scaling
> >factor to me. i.e. something like:
> >
> >
> >     if (!multidisk) {
> >             if (dblocks >= TERABYTES(4, blocklog)) {
> >                     blocks = XFS_AG_MAX_BLOCKS(blocklog);
> >                     goto done;
> >             }
> >             agcount = 4;
> >             /* work out ag size here */
> >             goto done;
> >     }
> >
> >I'd also like to see some test results showing the mkfs output
> >for the different configurations to confirm it works correctly
> >(i.e. that the corner cases work correctly).
> Dave, when this is put into place do you recommend people re-format their 
> XFS partitions for those with a 750GiB drive -or- with a < 2TB RAID5 
> array,

No. If you are having performance problems, then changing the way the
filesystem is laid out *may* improve performance but if everything
is working fine then don't change it.

> would one see any increase in speed?

On a single disk, yes. On RAID5 - who knows. There are so many other
variables to raid5 performance (esp software raid) that such single disk
optimisations could degrade performance. On other RAID hardware, it
might improve - it really depends on the RAID implementation....


Dave Chinner
Principal Engineer
SGI Australian Software Group

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