On Wed, Sep 02, 2009 at 12:13:15PM -0400, Bill Davidsen wrote: > Keld Jørn Simonsen wrote: >> On Sun, Aug 23, 2009 at 04:24:39PM +0200, Piergiorgio Sartor wrote: >> >>> Hi all, >>> >>> some time ago I was reporting about a strange issue. >>> >>> I have a two HDDs system, with a small RAID1 (/boot) >>> and the rest as RAID10 f2 (with LVM on top). >>> >>> It seems that /dev/sdb has more reads than /dev/sda. >>> >>> I had a quick check, with "iostat", and it seems that >>> all small reads, somehow below 1~4KiB, are done from >>> /dev/sdb2, regardless. >>> Actually, it seems that only if there is a pending >>> (small) read, this will be scheduled to /dev/sda2, >>> but non-overlapping small reads seem to happen always >>> from /dev/sdb2. >>> >>> This occurs with the RAID10, but it seems also with >>> the RAID1. >>> >> >> Hmm, have you done testing separately on each array? >> >> >>> In normal operation, this does not seem to lead to >>> problems, but during the smart long test /dev/sdb >>> takes by far more time than /dev/sda, since each >>> small read stop the test, and small read occurs >>> whenever there is a small write from syslog or >>> similar. >>> Note that failing and removing /dev/sdb2 results >>> in much shorter time for the smart test, about >>> 1hr30min vs. the 6~7hrs with the drive still >>> attached to RAID10. >>> >>> Is there any way to tune which is the "preferred" >>> drive or the "preferred" policy in case of these >>> small (or big) reads? >>> >> >> What level of the kernel are you running? >> >> >>> Could this be due to HW configuration? >>> The two HDDs are numbered SATA1 and SATA2 in BIOS, >>> there are still SATA3 and SATA4 ports somehow >>> available (SATA3 has a DVD). >>> >>> How are the reads scheduled withing the RAID10 software? >>> >> >> there was a change of this about 2.6.25 which forced reads to always be >> from the faster inner part of the disks, and that should even out reads. >> > > ??? what faster inner part is that? The linear velocity and > bytes/cylinder are higher as the diameter increases. I meant the faster lower numbered sectors, which are the *outer* sectors on the physical disk. Best regards Keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
