From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: First RAID Setup Date: Thu, 22 Dec 2005 09:20:22 -0500 Message-ID: <43AAB626.3060708@tmr.com> References: <0481A62A3E95A044AC20A1A45899645C06B02F4B@glc-mail-1.tessco.com> <43A1D642.2010301@wasp.net.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <43A1D642.2010301@wasp.net.au> Sender: linux-raid-owner@vger.kernel.org To: Brad Campbell Cc: "Callahan, Tom" , 'Andargor The Wise' , linux-raid@vger.kernel.org List-Id: linux-raid.ids Brad Campbell wrote: > Callahan, Tom wrote: > >> It is always wise to build in a spare however, that being said about all >> raid levels. In your configuration, if a disk fails in your RAID5, your >> array will go down. RAID5 is usually 3+ disks, with a mirror. So you >> should >> have 3 disks at minimum, and then a 4th as a spare. >> > > /me wonders in the days of reliable RAID-6 why we use RAID-5 + spare? > > RAID-6 has saved me twice now from dual drive failures on a 15 disk > array. > It's schweeeeeeeeeett It's also a lot more overhead... RAID-5 needs to update just one parity block beyond the data written. As I understand the Q sum in RAID-6, and watching disk access rates, each write requires the entire stripe to be read, then P and Q calculated, then written. You can do the P with a read+write, but since you have to read the entire stripe for Q, you save a read by recalculating the P from data. Did I say that right, Neil? If you are seeing dual drive failures, I suspect your hardware has problems. We run multiple 3 and 6 TB databases, and over a dozen 1 TB data caching servers, all using a lot of small fast disk, and I haven't seen a real dual drive failure in about 8 years. We did see some cases which looked like dual failures, it turned out to be a firmware limitation, controller not waiting for the bus to settle after a real failure, and thinking the next i/o had failed (or similar, in any case a false fail on the transaction after the real fail). If you run two PATA drives on the same cable in master/slave, it's at least possible that this could happen with consumer grade hardware as well. Just a thought, dual failures are VERY unlikely unless one triggers the other in some way, like failing the bus or cabinet power supply. -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979