2.6.20 XFS and LSI8888ELP race with 8 Virtual drives

2.6.20 XFS and LSI8888ELP race with 8 Virtual drives

[Mr. Berkley Shands]

I have a uniwide 3346, 20GB 8-core Opteron (2.2GHz)
With one LSI-8888ELP PCI-e SAS controller.
Each of the two external mini-sas 4-lane connectors goes to
an Xtore XJ-SAxx-316J-2 (16 drive 3U with dual SAS ports).
Running both Centos 4.4 or RedHat-5 Server with kernel 2.6.20.

The raids are all 4 drive raid0, 128KB stripes, using Seagate 7200.10
320GB drives (all firmware AAJ or later) Sata-II drives.
XFS filesystem, properly aligned, mkfs'ed etc.

If I write to all 4 VDs on one Xtore at a time, I get ~550MB/Sec
combined write rate. If I try writing to 7 of the 8 VDs at one time,
I get just under 800MB/Sec. Any 7 of 8 VDs.
But If I try to write to ALL 8 at once, the system crawls to a stop,
and I get Micro-Bytes per second.

xfs_datad/<N> process runs every few seconds, then pdflush runs
two or three at a time every few seconds. But iostat reports 0 writes.
every 10 seconds or so (iostat 5) reports a burst of write activity,
and then 10 seconds of nothing.

If I use  the Adaptec 4805 (two of them), I get about
the same write rate, but it does not hang (AACRAID driver).
same type of file systems, same external enclosures.

I had a "echo 10 > /proc/sys/vm/dirty_ratio" fix in to correct slow
writes (vs 2.6.18), but altering this value changed nothing. It still
runs oh-so-slow.

What to look at next? 2.6.21-rc4 ?

berkley
-- 

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

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