Re: XFS Tunables for High Speed Linux SW RAID5 Systems?

[David Chinner <dgc@sgi.com>]

On Fri, Jun 15, 2007 at 04:36:07PM -0400, Justin Piszcz wrote:
> Hi,
> 
> I was wondering if the XFS folks can recommend any optimizations for high 
> speed disk arrays using RAID5?

[sysctls snipped]

None of those options will make much difference to performance.
mkfs parameters are the big ticket item here....


> There is also vm/dirty tunable in /proc.

That changes benchmark times by starting writeback earlier, but
doesn't affect actual writeback speed.

> I was wondering what are some things to tune for speed?  I've already 
> tuned the MD layer but is there anything with XFS I can also tune?
> 
> echo "Setting read-ahead to 64MB for /dev/md3"
> blockdev --setra 65536 /dev/md3

Why so large? That's likely to cause readahead thrashing problems
under low memory....

> echo "Setting stripe_cache_size to 16MB for /dev/md3"
> echo 16384 > /sys/block/md3/md/stripe_cache_size
> 
> (also set max_sectors_kb) to 128K (chunk size) and disable NCQ

Why do that? You want XFS to issue large I/Os and the block layer
to split them across all the disks. i.e. you are preventing full
stripe writes from occurring by doing that.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group