From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?UTF-8?B?S2Fyc3RlbiBSw7Zta2U=?= Subject: Re: misunderstanding of spare and raid devices? - and one question more Date: Thu, 30 Jun 2011 16:05:51 +0200 Message-ID: <4E0C82BF.6050206@gmx.de> References: <4E0C5539.4030000@gmx.de> <4E0C5E47.5090604@anonymous.org.uk> <4E0C6CC4.3030506@turmel.org> <4E0C7196.1070307@gmx.de> <4E0C7B4B.7090404@turmel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4E0C7B4B.7090404@turmel.org> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Hi Phil > > So the first layout is the one you wanted. Each drive is ~4GB ? Or is this just a test setup? It's not a test setup. Historical reasons. I started whith Linux around 1995 and use software raid a long time. So I have this 4GB partition a long time and when I decide to upgrade storage or a hd says goodby, I use a new 4GB partition... Later I put more raid-arrays under a lvm, so I have no trouble with space on a single partition. >> Are there any experiences in which percentage the performance penalty is to expect? > > I don't have percentages to share, no. They would vary a lot based on number of disks > and type of CPU. As an estimate though, you can expect raid6 to be about as fast as > raid5 when reading from a non-degraded array. Certain read workloads could even be faster, > as the data is spread over more spindles. It will be slower to write in all cases. > The extra "Q" parity for raid6 is quite complex to calculate. In a single disk failure situation, > both raid5 and raid6 will use the "P" parity to reconstruct the missing information, so > their single-degraded read performance will be comparable. With two disk failures, > raid6 performance plummets, as every read requires a complete inverse "Q" solution. > Of course, two disk failures in raid5 stops your system. So running at a crawl, with data intact, is better than no data. That's the reason to think about a spare disc > You should also consider the odds of failure during rebuild, which is a serious concern for large raid5 arrays. > This was discussed recently on this list: > > http://marc.info/?l=linux-raid&m=130754284831666&w=2 > If your CPU has free cycles, I suggest you run raid6 instead of raid5+spare. > > Phil > I think there are free cycles, so I should try it. Thanks Karsten