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. Anyway I have seen some strange test results on reading small blocks with raid10,f2 , but maybe this was pre 2.6.25. raid1 reads not behaving as expected has been reported before. 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
