Writing performance problem with SAS1068

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/

# dd if=/dev/zero of=/dev/sdj bs=4096 count=65536 conv=fdatasync
65536+0 records in
65536+0 records out
268435456 bytes (268 MB) copied, 2.24953 seconds, 119 MB/s

# 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, 2.3246 seconds, 115 MB/s

Both /dev/sdj and /dev/sdd connect via an expander to the same SAS disk. /dev/sdj is via the LT aic94xx driver and a PCI-X HBA. /dev/sdd is via the mptsas driver and a SAS1068 (PCIe) based HBA. The kernel version is 2.6.20-rc5.

Looks good to me.

You may like to check that Write Cache Enable is on with:
'sdparm --get=WCE /dev/sdd'.

Doug Gilbert

Yeah, yeah! Works fine once I toggled the WCE to 1. Writing flies at 70MB/s, which is extremely good for those desktop-grade disks (Seagate Barracuda SATA 250GB @7200RPM).

Thank you very much!

But... who do you think I should bug to make this the system default? Does write caching need to be enabled by the driver itself, in the SCSI layer or perhaps by the distro initscripts?

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