From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: RAID5E Date: Mon, 12 Jun 2006 09:50:06 -0400 Message-ID: <448D710E.2020100@tmr.com> References: <447D9E72.3000704@tmr.com> <17534.16158.788542.538484@cse.unsw.edu.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <17534.16158.788542.538484@cse.unsw.edu.au> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Linux Raid List List-Id: linux-raid.ids Neil Brown wrote: >On Wednesday May 31, davidsen@tmr.com wrote: > > >>Where I was working most recently some systems were using RAID5E (RAID5 >>with both the parity and hot spare distributed). This seems to be highly >>desirable for small arrays, where spreading head motion over one more >>drive will improve performance, and in all cases where a rebuild to the >>hot spare will avoid a bottleneck on a single drive. >> >>Is there any plan to add this capability? >> >> > >I thought about it briefly.... > >As I understand it, the layout of raid5e when non-degraded is very >similar to raid6 - however the 'Q' block is simply not used. >This would be trivial to implement. > >The interesting bit comes when a device fails and you want to rebuild >that distributed spare. >There are two possible ways that you could do this: > >1/ Leave the spare where it is and write the correct data into each > spare. This would be fairly easy but would leave an array with an > very ... interesting layout of data. > When you add a replacement you just move everything back. > >2/ reshape the array to be a regular raid5 layout. This would be hard > to do well without NVRAM as you are moving live data, but would result > in a neat and tidy array. Ofcourse adding a drive back in would be > interesting again... > >I had previously only thought of option '2', and so discarded the idea >as not worth the effort. The more I think about it, the more possible >option 1 sounds. >I've put it back on my todo list, but I don't expect to get to it this >year. Ofcourse if someone else wants to give it a try, I'm happy to >make suggestions and review code. > I do appreciate being too busy, I'm just glad I have been able to clarify the tradeoffs of RAID5e, and get it on your list at all. I did look at the code a bit, and it would seem that if the "rebuild to hot spare" code is modified to handle a distributed spare, then it looks as if RAID6e might pretty much fall out. Feel free to tell me I'm dreaming. -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979