From mboxrd@z Thu Jan 1 00:00:00 1970 From: Justin Piszcz Subject: Re: which raid level gives maximum overall speed? (raid-10,f2 vs. raid-0) Date: Tue, 5 Feb 2008 17:28:27 -0500 (EST) Message-ID: References: <20080130192133.17b254bf@szpak> <20080130220007.GG20173@rap.rap.dk> <20080130233639.5d46b044@szpak> <20080131015506.GB6617@rap.rap.dk> <20080205161036.GA30326@rap.rap.dk> <20080205200423.GB17621@rap.rap.dk> Mime-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="-1463747160-1114481314-1202250507=:10352" Return-path: In-Reply-To: <20080205200423.GB17621@rap.rap.dk> Sender: linux-raid-owner@vger.kernel.org To: =?ISO-8859-15?Q?Keld_J=F8rn_Simonsen?= Cc: Janek Kozicki , linux-raid@vger.kernel.org List-Id: linux-raid.ids This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. ---1463747160-1114481314-1202250507=:10352 Content-Type: TEXT/PLAIN; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE On Tue, 5 Feb 2008, Keld J=F8rn Simonsen wrote: > On Tue, Feb 05, 2008 at 11:54:27AM -0500, Justin Piszcz wrote: >> >> >> On Tue, 5 Feb 2008, Keld J=F8rn Simonsen wrote: >> >>> On Thu, Jan 31, 2008 at 02:55:07AM +0100, Keld J=F8rn Simonsen wrote: >>>> On Wed, Jan 30, 2008 at 11:36:39PM +0100, Janek Kozicki wrote: >>>>> Keld J=F8rn Simonsen said: (by the date of Wed, 30 Jan 2008 23:00= :07 >>>>> +0100) >>>>> >>>> >>>> All the raid10's will have double time for writing, and raid5 and raid= 6 >>>> will also have double or triple writing times, given that you can do >>>> striped writes on the raid0. >>> >>> For raid5 and raid6 I think this is even worse. My take is that for >>> raid5 when you write something, you first read the chunk data involved, >>> then you read the parity data, then you xor-subtract the data to be >>> changed, and you xor-add the new data, and then write the new data chun= k >>> and the new parity chunk. In total 2 reads and 2 writes. The read/write= s >>> happen on the same chunks, so latency is minimized. But in essence it i= s >>> still 4 IO operations, where it is only 2 writes on raid1/raid10, >>> that is only half the speed for writing on raid5 compared to raid1/10. >>> >>> On raid6 this amounts to 6 IO operations, resulting in 1/3 of the >>> writing speed of raid1/10. >>> >>> I note in passing that there is no difference between xor-subtract and >>> xor-add. >>> >>> Also I assume that you can calculate the parities of both raid5 and >>> raid6 given the old parities chunks and the old and new data chunk. >>> If you have to calculate the new parities by reading all the component >>> data chunks this is going to be really expensive, both in IO and CPU. >>> For a 10 drive raid5 this would involve reading 9 data chunks, and >>> making writes 5 times as expensive as raid1/10. >>> >>> best regards >>> keld >>> - >>> 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 >>> >> >> On my benchmarks RAID5 gave the best overall speed with 10 raptors, >> although I did not play with the various offsets/etc as much as I have >> tweaked the RAID5. > > Could you give some figures? I remember testing with bonnie++ and raid10 was about half the speed=20 (200-265 MiB/s) as RAID5 (400-420 MiB/s) for sequential output, but input= =20 was closer to RAID5 speeds/did not seem affected (~550MiB/s). Justin. ---1463747160-1114481314-1202250507=:10352--