From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: New raid level suggestion. Date: Thu, 30 Dec 2010 18:10:48 +0000 Message-ID: <4D1CCB28.8030705@anonymous.org.uk> References: <20101230082356.GC2986@bitwizard.nl> <4D1C470E.4080406@crc.id.au> <20101230094230.GE2986@bitwizard.nl> <4D1C616D.8030904@hardwarefreak.com> <4D1C73D4.6050600@anonymous.org.uk> <4D1C851B.3040304@hardwarefreak.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4D1C851B.3040304@hardwarefreak.com> Sender: linux-raid-owner@vger.kernel.org To: Stan Hoeppner Cc: Linux RAID List-Id: linux-raid.ids On 30/12/2010 13:11, Stan Hoeppner wrote: > John Robinson put forth on 12/30/2010 5:58 AM: >> On 30/12/2010 10:39, Stan Hoeppner wrote: >> [...] >>> Any RAID scheme that uses parity is less than optimal, and up to >>> horrible, for heavy random IO loads. As always, this depends on "how >>> heavy" the load is. For up to a few hundred constant IOPS you can get >>> away with parity RAID schemes. If you need a few thousand or many >>> thousand IOPS, better stay away from parity RAID. >> >> Sorry, I have to disagree with this, in this situation. RAID-6 over 4 >> discs will be just as fast for reading multiple small files as RAID-10 >> over 4 discs, and a web server is a read-mostly environment, while at >> the same time I can't imagine any RAID schema ever giving thousands of >> IOPS over 4 discs, parity or no. > > That's because you apparently didn't learn about paragraph's in English > class: http://en.wikipedia.org/wiki/Paragraph Do you Brits use > paragraphs differently than we do here in the states? No, but apparently we use apostrophes correctly over here. > My first paragraph dealt with general performance of parity vs non > parity RAID WRT high IO loads. Yes, and I suppose that I should have pointed out that the OP's friend had been given slightly inappropriate advice, since a web server doesn't do small file I/O like a mailserver. You expanded on a general situation which didn't apply, and the statement you made was wrong, or at least not correct in all circumstances. > My second paragraph covered the downside > of the redundancy methods of RAID 3/4. You were wrong again there: if you lose the parity disc in RAID 3/4 you don't lose the array, as the data discs are all still there. It is true that with modern huge (1TB+) drives where the error rate per bit read is still much the same as when drives were tiny (1GB+) that a recovery is much more risky than it used to be due to the dramatically increased chance of a second disc failing, but that is equally true of RAID 5. > My third paragraph dealt > specifically with Roger's web server. The third and the fourth; jolly good. > Note that nothing in my first paragraph mentioned a web server workload. > Also note that nowhere did I mention a count of 4 drive, nor commented > regarding the suitability of any RAID level with 4 drives. No indeed, but that was the context of the question; why give entirely general advice when a specific usage applies? > Also note there were two "situations" mentioned by Roger. The first > referenced a previous thread which dealt with a high transaction load > server similar to a mail server, IIRC. I see no such reference, apart from noting that "when asking for help, everybody pounced on us: - NEVER use raid5 for a server doing small-file-io like a mailserver. (always use RAID10)" which as I say is in my opinion inappropriate advice, since they're not trying to run a mailserver and won't have heavy random writes. > My first paragraph related to > that. The second "situation", to which you refer, dealt with Roger's > web server. I had surmised from the original question about using RAID-10, RAID-4 etc that there was a desire to have more storage than a single drive mirrored twice, so I didn't think plain mirroring would suit, but perhaps that wasn't the intention and your solution would work. Cheers, John.