bloch@xxxxxxxxxxxx wrote: > 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: Using 'lsscsi -g' would also give you the generic device numbers. It is interesting that the above "ATA" disks do not have corresponding /dev/sd* device names. > # 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. Good. > 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. The mechanism for doing this was only formalized recently with the SAT standard, so it may take a while for BIOSes and other infrastructure to catch up. Doug Gilbert - 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
