From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: raid10 vs raid5 - strange performance Date: Wed, 26 Mar 2008 12:29:05 -0400 Message-ID: <47EA79D1.1090607@tmr.com> References: <72dbd3150803251513v28ea142ct2de229c26e888c56@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Christian Pernegger Cc: David Rees , Linux RAID List-Id: linux-raid.ids Christian Pernegger wrote: >> Not surprising at all. Read performance is similar between the two >> setups as expected (appears to be limited by the PCI bus). >> > > Yes, reads are fine. > > >> Streaming write performance is better because you are writing less >> redundant data to disks, you can now stripe writes over 5 disks >> instead of 3. >> > > Sounds reasonable. Write performance SHOULD be ~5x single disk for > raid5 and ~3x single disk for raid10 in a theoretical best-case > scenario, either should hit the PCI bus cap. In reality the ratios are > more like 1x for raid5 and 0.6 -1.1x for raid10. It's just that I'd > expected ~ identical and significantly better streaming write > performance from both array configurations ... then again, if you > think it over, > > raid5 will transfer 1 parity block per 5 data blocks (so 5/6 of the > PCI bw are usable = 92MB) > raid10 will transfer 3 copy blocks per 3 data blocks (so 1/2 of the > PCI bw are usable = 55MB) > > Factoring in some contention / overhead my values might well be > normal. It just means that the fabled raid10 only performs if you have > high-bw buses, which this box sadly doesn't, or a hw controller where > redundant blocks don't go over the bus, which the 3ware 7506 > apparently isn't. > > I'll still go with raid10 for the 50% better random I/O, only less > enthusiastically. > I think that you should treat 10,n2 and 10,f2 as separate configurations, and test them as such. behavior is quite different, and one or the other might be a better fit to your usage. The ability to transfer a single copy of the data and no parity information is an advantage of hardware controllers. -- Bill Davidsen "Woe unto the statesman who makes war without a reason that will still be valid when the war is over..." Otto von Bismark