From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Unbalanced reads of RAID10 Date: Wed, 02 Sep 2009 12:13:15 -0400 Message-ID: <4A9E999B.8070401@tmr.com> References: <20090823142439.GA5162@lazy.lzy> <20090830003706.GB5633@rap.rap.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20090830003706.GB5633@rap.rap.dk> Sender: linux-raid-owner@vger.kernel.org To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= Cc: Piergiorgio Sartor , linux-raid@vger.kernel.org List-Id: linux-raid.ids Keld J=F8rn Simonsen wrote: > On Sun, Aug 23, 2009 at 04:24:39PM +0200, Piergiorgio Sartor wrote: > =20 >> 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. >> =20 > > Hmm, have you done testing separately on each array? > > =20 >> 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? >> =20 > > What level of the kernel are you running? > > =20 >> 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? >> =20 > > 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 rea= ds. > =20 ??? what faster inner part is that? The linear velocity and=20 bytes/cylinder are higher as the diameter increases. --=20 bill davidsen CTO TMR Associates, Inc "Now we have another quarterback besides Kurt Warner telling us during = postgame interviews that he owes every great thing that happens to him on a foot= ball field to his faith in Jesus. I knew there had to be a reason why the Al= mighty included a mute button on my remote." -- Arthur Troyer on Tim Tebow (Sports Illustrated) -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html