On Tue, 05 Jun 2007, bloch@xxxxxxxxxxxx wrote: > Hello > > I'm seeing very slow writes on a Dell Precision 690 with the Dell SAS5 > adapter, serving a RAID1 array of SATA-II disks. > > It's very similar to the problem in FreeBSD, described here: > > http://unix.derkeiler.com/Mailing-Lists/FreeBSD/stable/2007-03/msg00756.html > > I'm running FC6 with the latest kernel. > > Reads are quite fast, writes terribly slow. > Thanks to all who replied to this query, especially the very detailed response from Eric Moore at LSI. The first important facet is that we need to operate on the two hidden physical disks, not the RAID device. lsscsi differentiates them: # lsscsi [0:0:0:0] disk ATA WDC WD5000KS-75M 2E08 - [0:0:1:0] disk ATA HDS725050KLA360 AB5A - [0:1:0:0] disk Dell VIRTUAL DISK 1028 /dev/sda sg_map gives the generic device numbers: # sg_map -i -x /dev/sg0 0 0 0 0 0 ATA WDC WD5000KS-75M 2E08 /dev/sg1 0 0 1 0 0 ATA HDS725050KLA360 AB5A The write cache can then be enabled using sdparm: sdparm -s WCE=1 -S /dev/sg0 and the result checked with # sdparm -g WCE /dev/sg1 /dev/sg1: ATA HDS725050KLA360 AB5A WCE 1 [cha: y] This seems to make the write performance much better. The question for Dell is why their version of the BIOS doesn't set the write cache in the first place or allow it to be altered by the user. Cheers, Adam - 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
