From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Joachim Otahal (privat)" Subject: Re: RAID5 with two drive sizes question Date: Tue, 05 Jun 2012 22:48:13 +0200 Message-ID: <4FCE708D.2080903@gmx.net> References: <4FCE4199.7030705@gmx.net> <20120605233953.2086cc4c@natsu> <4FCE60F3.4040606@gmx.net> <20120606015950.585d5454@natsu> <4FCE6DCD.7030207@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: <4FCE6DCD.7030207@hardwarefreak.com> Sender: linux-raid-owner@vger.kernel.org To: Mdadm List-Id: linux-raid.ids Stan Hoeppner schrieb: > On 6/5/2012 2:59 PM, Roman Mamedov wrote: >> On Tue, 05 Jun 2012 21:41:39 +0200 >> "Joachim Otahal (privat)" wrote: >> >>> Use only 750GB partitions, use the 3*250 GB loss at the end of each 1 TB >>> drive for the fourth 750 GB, and RAID6 those 8*750. Result is 4.5 TB >>> with a one-drive-loss tolerance and really bad performance. >>> I spare you the 500 GB partitions example which result in 4.5 TB with a >>> one-drive-loss tolerance and really bad performance. >> Except this would not make any sense even as a thought experiment. You don't >> want a configuration where two or more areas of the same physical disk need to >> be accessed in parallel for any read or write to the volume. And it's pretty >> easy to avoid that. > You make a good point but your backing argument is incorrect: XFS by > design, by default, writes to 4 equal sized regions of a disk in parallel. > > The problem here is running multiple RAID arrays, especially of > different RAID levels, on the same physical disk. Under high IO load > you end up thrashing the heads due to excessive seeking as the access > patterns are very different between the arrays. In some situations it > may not cause problems. In others it can. > > For a home type server with light IO load you probably won't have any > problems. For anything with a high IO load, you don't want to do this > type of RAID setup. Anyone with such an IO load already knows this, > which is why it's typically only hobbyists who would consider using such > a configuration. > Please stop. Next time I use tags. RAID5 with 1 TB packages and appending the remaining 2*500 GB as RAID1 (as suggested by Roman Mamedov) is indeed the only sensible way, everything else is nonsense.