On Wed, 7 Nov 2012 22:47:20 +1100 Geoff Attwater <geoffwater@xxxxxxxxx> wrote: > I have a relatively unimportant home fileserver that uses an mdadm > raid5 across three 1TB partitions (on separate disks - one is 1.5 TB > and has another 500GB partitition for other stuff). I wish to convert > it to raid10 across 4 1TB partitions by adding a fresh drive. > > The mdadm man page, section *Grow Mode* states that it may > > "convert between RAID1 and RAID5, between RAID5 and RAID6, between > RAID0, RAID4, and RAID5, and between RAID0 and RAID10 (in the near-2 > mode)." > > Conversion between RAID5 and RAID10 directly is not supported (mdadm > tells you so if you try it). > So my plan was to do a three stage conversion: > > 1. back everything up > 2. convert from 3-disk raid5 -> 2-disk raid0 (now with no redundancy, > but it's backed up, so that's ok) > 3. convert the 2-disk raid0 -> 4-disk raid10 > > All of these have the same logical size (2TB). This is on an Ubuntu > 12.10 system. > mdadm --version reports: > mdadm - v3.2.5 - 18th May 2012 > uname -a reports: > Linux penguin 3.5.0-18-generic #29-Ubuntu SMP Fri Oct 19 10:26:51 UTC > 2012 x86_64 x86_64 x86_64 GNU/Linux > > I searched around to see if anyone had followed this kind of procedure > before, but didn't find anything directly addressing exactly what I > was trying to do (I saw much more about raid0 -> raid5 type > conversions, while adding a device and the like and nothing much on > going the other way), so I proceeded based on what I understood from > the man page and other general stuff on mdadm raid reshaping I read. > > for stage 2, I used the command > > mdadm --grow /dev/md0 --level=raid0 --raid-devices=2 > --backup-file=/media/newdisk/raid_to_0_backup > > where the backup-file is on another disk not in the array. I put the > --raid-devices=2 in to make it clear that what I was after was 2x1TB > disks in RAID0 and one spare (the same logical size), rather than a > larger logical size 3TB three-disk RAID0. Although based on Neil > Brown's blog post at http://neil.brown.name/blog/20090817000931 it > seems the conversion should generally operate by reshuffling things > into an equal-logical size array anyway, so that perhaps wasn't > necessary. > > This began a lengthy rebuild process that has now finished. However, > at the end of the process, after no visible error messages and > obviously a lot of data movement seen via iostat, `mdadm --detail > /dev/md0` showed the array as *still raid5* with all disks used, and > the dmesg output contained these relevant lines: > > [93874.341429] md: reshape of RAID array md0 > [93874.341435] md: minimum _guaranteed_ speed: 1000 KB/sec/disk. > [93874.341437] md: using maximum available idle IO bandwidth (but > not more than 200000 KB/sec) for reshape. > [93874.341442] md: using 128k window, over a total of 976630272k. > === snip misc unrelated stuff === > [183629.064361] md: md0: reshape done. > [183629.072722] RAID conf printout: > [183629.072732] --- level:5 rd:3 wd:3 > [183629.072738] disk 0, o:1, dev:sda1 > [183629.072742] disk 1, o:1, dev:sdc1 > [183629.072746] disk 2, o:1, dev:sdb1 > [183629.088584] md/raid0:md0: raid5 must be degraded! Degraded disks: 0 > [183629.091657] md: md0: raid0 would not accept array These last two are the interesting messages. The raid0 module in the kernel will only access a raid5 for conversion if it is in 'parity-last' layout, and is degraded. But it isn't. mdadm should fail and remove the 'parity' disk before trying to convert to raid0, but it doesn't. I guess I never tested it - and untested code is buggy code! You could be able to finish the task manually. - fail the last (parity) device - remove that device - echo raid0 > /sys/block/md0/md/level So: mdadm /dev/md0 -f /dev/sdb1 mdadm /dev/md0 -r /dev/sdb1 echo raid0 > /sys/block/md0/md/level However you should double-check that 'sdb1' is the correct device. Look in the output of 'mdadm -D' and see what raid device number '2' is. I'll add this to my list of things to fix. Thanks, NeilBrown
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