Bernardo Innocenti wrote: > 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 # 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 > *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. > - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
