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 11:54:27 -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> Mime-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="-1463747160-1449706023-1202230467=:9790" Return-path: In-Reply-To: <20080205161036.GA30326@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. ---1463747160-1449706023-1202230467=:9790 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 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 23:00:0= 7 +0100) >>> >> >> All the raid10's will have double time for writing, and raid5 and raid6 >> 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 chunk > and the new parity chunk. In total 2 reads and 2 writes. The read/writes > happen on the same chunks, so latency is minimized. But in essence it is > 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" in > 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,=20 although I did not play with the various offsets/etc as much as I have=20 tweaked the RAID5. Justin. ---1463747160-1449706023-1202230467=:9790--