From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Haigh Subject: Re: New raid level suggestion. Date: Thu, 30 Dec 2010 19:47:10 +1100 Message-ID: <4D1C470E.4080406@crc.id.au> References: <20101230082356.GC2986@bitwizard.nl> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20101230082356.GC2986@bitwizard.nl> Sender: linux-raid-owner@vger.kernel.org To: Rogier Wolff Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 30/12/2010 7:23 PM, Rogier Wolff wrote: > > Hi, > > A friend has a webserver. He has 4 drive bays and due to previous > problems he's not content to have 3 or 4 drives in a raid5 > configuration, but he wants a "hot spare" so that when it takes him a > week to find a new drive and some time to drive to the hosting > company, he isn't susceptible to a second drive crashing in the > meantime. > > So in principle he'll build a 3-drive RAID5 with a hot spare.... > > Now we've been told that raid5 performs badly for the workload that is > expected. It would be much better to run the system in RAID10. However > if he'd switch to RAID10, after a single drive failure he has a window > of about a week where he has a 33% chance of a second drive failure > being "fatal". > > So I was thinking.... He's resigned himself to a configuration where > he pays for 4x the disk space and only gets 2x the available space. > > So he could run his array in RAID10 mode, however when a drive fails, > a fallback to raid5 would be in order. In this case, after the resync > a single-drive-failure tolerance is again obtained. > > In practise scaling down to raid5 is not easy/possible. RAID4 however > should be doable. > > In fact this can almost be implemented entirely in userspace. Just > remove the mirror drive from the underlying raid0, and reinitialize as > raid4. If you do this correctly the data will still be there.... > > Although doing this with an active filesystem running on these drives > is probably impossible due to "device is in use" error messages.... > > So: Has anybody tried this before? > Can this be implemented without kernel support? > Anybody feel like implementing this? > > Roger. > Maybe I'm not quite understanding right, however you can easily do RAID6 with 4 drives. That will give you two redundant, effectively give you RAID5 if I drive fails, and save buttloads of messing around... -- Steven Haigh Email: netwiz@crc.id.au Web: http://www.crc.id.au Phone: (03) 9001 6090 - 0412 935 897 Fax: (03) 8338 0299