From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roberto Spadim Subject: Re: Mirrored volume peformance questions Date: Wed, 4 May 2011 12:43:24 -0300 Message-ID: References: <96B30EF4F3E17749BCFF7F1816090EDA02DCE41A87@EX-SEA31-B.ant.amazon.com> <20110503213430.GC24265@www2.open-std.org> <20110504081339.GB414@www2.open-std.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110504081339.GB414@www2.open-std.org> Sender: linux-raid-owner@vger.kernel.org To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= Cc: David Brown , linux-raid@vger.kernel.org List-Id: linux-raid.ids i think that a good add to today read balance could be add disk acess_time (1/rpm, 0 for 'non rotational', 1 for unknow) head_distance * acess_time, could allow a better select of disks or mix of 7200/10000/15000 rpm disks maybe it can be read from disk information at assemble time, i don't know if disk speed (rpm) can be read from device block (some disks display information with hdparm and smartctl) it's not a 1000% improvement, it's 1% at high read/write load 2011/5/4 Keld J=F8rn Simonsen : > On Wed, May 04, 2011 at 09:42:40AM +0200, David Brown wrote: >> raid10,far is better for sequential reads - it gives better-than-rai= d0 >> performance on average since it will do striped reads from the faste= r >> outer tracks. =A0And for multi-threaded reads, it should also be a l= ittle >> faster than other raid10 layouts (and raid1, which is much the same = as >> raid10,near). =A0Since it prefers to get the data from the outer hal= f, you >> get the benefits of short-stroking your disks - faster transfer spee= ds >> and less head movement. >> >> The cost of raid10,far is greater head movement for writes - but tha= t is >> not the OP's main concern. > > yes, in theory this is so. But two reasons almost eliminates this in > practice. First, the processes do not wait for completion of the IO o= f > writes, the processes only deliver the data to the file buffer cache = of > the kernel, which then periodically flushes the data to the disk driv= es. > Second, the flushing of the data is ordered so that the collected dat= a > buffers are written as much sequentially as possible to the drives. > This goes for all Linux MD RAID1/RAID10 layouts. Given that random > writes are random over the whole set of drives, for any mirrored > raid1/raid10 layout, the flushing of the data is about the same. > > best regards > keld > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid"= in > the body of a message to majordomo@vger.kernel.org > More majordomo info at =A0http://vger.kernel.org/majordomo-info.html > --=20 Roberto Spadim Spadim Technology / SPAEmpresarial -- 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