XFS and 2.6.18 -> 2.6.20-rc3

["Mr. Berkley Shands" <bshands@exegy.com>]

My testbench is a 4 core Opteron (dual 275's) into
two LSI8408E SAS controllers, into 16 Seagate 7200.10 320GB satas.
Redhat ES4.4 (Centos 4.4). A slightly newer parted is needed
than the contemporary of Moses that is shipped with the O/S.

I have a standard burn in script that takes the 4 4-drive raid0's
and puts a GPT label on them, aligns the partitions to stripe
boundary's. It then proceeds to write 8GB files concurrently
onto all 4 raid drives.

Under 2.6.18.1 the write speeds start at 265MB/Sec and decrease
mostly monotonically down to ~160MB/Sec, indicating that
the files start on the outside (fastest tracks) and work in.
All 4 raids are within 7-8MB/Sec of each other (usually they
are identical in speed).

By the time of 2.6.20-rc3, the same testbench shows
a 10% across the board decrease in throughput for writes.
Reads are unaffected.
But now the allocation order for virgin file systems are random,
usually starting at the slow 140MB/Sec, then bouncing up to 220MB/Sec,
then around and around. No two raids get the same write speeds at the 
same time.

Dave Lloyd (our in-house Idea Guy) looked at the allocation groups...
Non-sequential, random...

What data would you like to see?

The run logs from 2.6.18.1 and 2.6.20-rc3?
Want the scripts?
The xfs-debug dumps of a few files?

Berkley

-- 

//E. F. Berkley Shands, MSc//

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