On Sun, Aug 07 2016, Wols Lists wrote: > On 07/08/16 01:32, Glenn Enright wrote: >> On 7/08/2016 12:01 pm, "Wols Lists" <antlists@xxxxxxxxxxxxxxx >> <mailto:antlists@xxxxxxxxxxxxxxx>> wrote: >>> >>> On 05/08/16 21:16, Wols Lists wrote: >>> > In my testing of xosview, I've been mucking about with a vm and raid. >>> > xosview is looking quite promising (I've got a few comments about it, >>> > but never mind). >>> > >>> > BUT. In mucking about with raid 1, I increased my raid devices to three. >>> > I now just can NOT convert the array to raid 5! I've been mucking around >>> > with all sorts of things trying to get it to work, but finally two error >>> > messages make things clear. >>> > >>> Following up to myself - suddenly thought "I know what's wrong". So I >>> stopped the array, and of course couldn't access it, it was no longer >>> there. So I assembled but didn't run it, and it worked fine. >>> >>> Simples, once you realise what's wrong - you can ADD devices to a >>> running array, but you can't REMOVE them. >>> >>> Cheers, >>> Wol >>> > >> >> You can remove em if you mark em as failed first. Eg >> >> Mdadm /dev/mdx --fail /dev/sdc1 --remove /dev/sdc1 >> >> Best, Glenn >> > Except - if you read my original post - I was trying to TOTALLY remove > the device! > > mdadm --grow -raid-devices=2 > > THAT was the problem - I had a 3-device mirror, and you can't convert > that to raid5! Even if you've --fail --remove'd the third device! > > In other words, "--grow --raid-devices=more" will work on a running > device, "--grow --raid-devices=less" will only work on an array that is > built but not running. I don't believe this is correct, and I could reproduce your results in quick tests. If the array is not running, then you cannot reshape it at all. You can reduce the number of devices in a RAID1 at any time as long as the number of active devices is not greater than the number of devices requested. /dev/md0 has 3 working devices: # mdadm -G /dev/md0 -n2 mdadm: failed to set raid disks # mdadm /dev/md0 -f /dev/loop0 mdadm: set /dev/loop0 faulty in /dev/md0 # mdadm -G /dev/md0 -n2 raid_disks for /dev/md0 set to 2 > > I now have the problem that my "--grow --level=5" has fallen foul of the > "reshape stuck at zero" problem, and I can now neither run the array, > nor get the reshape working ... :-( We really need to fix this ... if only I knew how to reproduce it. NeilBrown
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