From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Subject: Re: raind-1 resync speed slow down to 50% by the time it finishes Date: Fri, 31 Jul 2009 19:54:55 +0200 Message-ID: <20090731175455.GA11463@rap.rap.dk> References: <20090730073554.GA17665@cthulhu.home.robinhill.me.uk> <20090730101846.GA17332@rap.rap.dk> <72dbd3150907301311t5abef2fai91cdae154b987d33@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <72dbd3150907301311t5abef2fai91cdae154b987d33@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: David Rees Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Thu, Jul 30, 2009 at 01:11:20PM -0700, David Rees wrote: > 2009/7/30 Keld J=F8rn Simonsen : > > I think raid10,f2 only degrades 10-20 % while raid1 can degrade as = much > > as 50 %. For writing it is about the same, given that you use a fil= e > > system on top of the raid. >=20 > Has anyone done any benchmarks of near vs far setups? Yes, there are a number of benchmarks on raid10 near/far scenarios at http://linux-raid.osdl.org/index.php/Performance > >From what I understand, here's how performance should go for a 2-dis= k > raid10 setup: >=20 > Streaming/large reads far: Up to 100% faster since reads are striped > across both disks and possibly faster, due to far only using the faster half of the disk for reading. > Streaming/large reads near: Same as single disk as reads can't be > striped across both disks yes. > Streaming/large writes far: Slower than single disk, since disks have > to seek to write. How much of a hit in performance will depend on > chunk size. > Streaming/large writes near: Same as single disk. Due to the elevator of the file system, writes are about the same for both near and far. > Random/small reads far: Up to 100% faster Actually a bit more, due to that far only uses the fastest half of the disks. One test shows 132 % faster, which is consistent with theory. > Random/small reads near: Up to 100% faster One test shows 156 % faster. > Random/small writes far: Same as single disk. > Random/small writes near: Same as single disk. yes. > So basically, if you have a setup which mostly reads from disk, using > a far layout is beneficial, but if you have a setup which does a > higher percentage of writes, sticking to a near layout will be faster= =2E =46or reading, this is true, but for writing, it is not true, given tha= t you use a filesystem, with an elevator algorithm in use. The elevator evens out the lesser performance of layout=3Dfar for a raw raid10,f2, s= o that the performance is about the same for the near and far layouts. > I recently set up an 8-disk RAID10 across 8 7200 disks across 3 contr= ollers. >=20 > 5 disks are in an external enclosure via eSATA and a PCIe card. > 2 disks are using onboard SATA controller > 1 disk is using onboard IDE controller >=20 > I debated whether or not to use near or far, but ultimately stuck wit= h > near for two reasons: >=20 > 1. The array mostly sees write activity, streaming reads aren't that = common. > 2. I can only get about 120 MB/s out of the external enclosure becaus= e > of the PCIe card [1] , so being able to stripe reads wouldn't help ge= t > any extra performance out of those disks. Hmm, a pci-e x1 should be able to get 2.5 Mbit/s =3D about 300 MB/s.=20 Wikipedia says 250 MB/s. It is strange that you only can get 120 MB/s. That is the speed of a PCI 32 bit bus. I looked at your reference [1] for the 3132 model. Have you tried it out in practice? The max you should be able to get out of your raid10 with 8 disks would then be around 400 - 480 MB/s, for sequential reads. 250 MB/s out of yo= ur PCIE enclosure, or 50 MB/s per disk, and then additional 50 MB/s each of the= last 3 disks. You can only multiply the speed of the slowest of the disks involved by the number of disks. But even then it is not so bad. =46or random read it is better yet, given that this is not limited by t= he transfer speed of your PCIe controller. > -Dave >=20 > [1] http://ata.wiki.kernel.org/index.php/Hardware,_driver_status#Sili= con_Image_3124 > -- > 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 http://vger.kernel.org/majordomo-info.html -- 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