Writing performance problem with SAS1068

[Bernardo Innocenti <bernie@develer.com>]

Hello,

I've stumbled onto a strange performance problem on a new server:
reading from disks is fast (70-80MB/s), but writing is extremely
slow (13-15MB/s).  I've measured it like this:

 dd if=/dev/zero of=/dev/sdd bs=4096 count=65536 conv=fdatasync
 65536+0 records in
 65536+0 records out
 268435456 bytes (268 MB) copied, 17.7004 seconds, 15.2 MB/s

*but*: if I rebuild the kernel and change CONFIG_FUSION_MAX_SGE
from 40 (Fedora's default) to 128 (maximum value), it suddenly
gets much faster: 31MB/s!

Looks very much like an interrupt problem to me.  Maybe
increasing the scatter gather mitigates the problem of
missing completion notifications.

Evidence:

 Exhibit A: custom kernel config for 2.6.18-1.2257.fc5.bernie
 http://www.codewiz.org/helium_logs/config

 Exhibit B: dmesg output from said kernel
 http://www.codewiz.org/helium_logs/dmesg

 Exhibit C: misc proc files, and all that
 http://www.codewiz.org/helium_logs/

 Exhibit D: motherboard and chipset specification
 http://www.supermicro.com/products/motherboard/Xeon3000/3010/PDSME+.cfm


Circumstantial evidence:

 - Seems to affect just the LSI SAS1068 PCI-X controller.
   The on-board AHCI controller writes very fast (>60MB/s)

 - I've seen a very similar writing bottleneck with a
   Promise TX4 SATA controller (not PCI-X) on a server with
   a similar motherboard (Supermicro with Mukilteo 3000).

 - Passing mpt_msi_enable=1 doesn't change anything

 - FreeBSD 6.2 is even slower: writes at 7MB/s

 - OpenSolaris is much, much slower... less than 1MB/s.

 - Windows Vista (rc something) writes at 90MB/s.  Too
   fast to believe, maybe dd from Cygwin is misbehaving.

-- 
   // Bernardo Innocenti - Develer S.r.l., R&D dept.
 \X/  http://www.develer.com/