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/
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