From mboxrd@z Thu Jan 1 00:00:00 1970 From: Goswin von Brederlow Subject: Re: Subject: [001/002 ] raid0 reshape Date: Tue, 26 May 2009 13:17:46 +0200 Message-ID: <87skisnmed.fsf@frosties.localdomain> References: <1241300764.5607.36.camel@raz> <18965.16276.485692.812516@notabene.brown> <87f94c370905211220n4e686a30ke452a56a2690d22b@mail.gmail.com> <87r5ydqsrd.fsf@frosties.localdomain> <18971.6239.413944.47540@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: In-Reply-To: <18971.6239.413944.47540@notabene.brown> (Neil Brown's message of "Tue, 26 May 2009 08:14:55 +1000") Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Goswin von Brederlow , Greg Freemyer , Dan Williams , raz ben yehuda , linux-raid@vger.kernel.org, Jacek Danecki , "Labun, Marcin" List-Id: linux-raid.ids Neil Brown writes: > On Monday May 25, goswin-v-b@web.de wrote: >> That really seems to scream for LVM to support more raid levels. It >> already has linear, raid0 and raid1 support (although I have no idea >> how device mapper raid1 compares to md raid1). > > Note that LVM (a suite of user-space tools) could conceivably use > md/raid1, md/raid5 etc. The functionality doesn't have to go in dm. > > Neil How would you do this? Worst case you can have a LV made up of totaly non linear PEs, meaning lots of 4MB (default PE size) big chunks in random order on random disks. Do you create a raid1/5 for each stripe? You surely run out of md devices. Create dm mappings for all stripe 0s, stripe 1s, stripe 2s, ... and then a raid1/5 over those stripe devices? What if the LV has segments with different raid configurations (number of disks in a stripe or even different levels)? Create a raid for each segment and then a dm mapping for a linear raid? You can get a flood of intermediate devices there. A /proc/mdstat with 200 entries would be horrible. iostat output would be totaly useless. ... MfG Goswin