Re: RAID 1 partition with hot spare shows [UUU] ?

[NeilBrown <neilb@suse.de>]

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On Wed, 18 Apr 2012 00:42:54 +0200 John Crisp <john@reetspetit.net> wrote:

> Hi,
> 
> Sorry to trouble people, and I am sure you have better things to do, but
> I can't find an answer to the following and don't know where else to go.
> 
> I have been struggling with this problem for weeks and having read all I
> can I still don't know the answer.
> 
> I have a server running a version of CentOS 5.x   Yes, mdadm is old at
> 2.6.9 but it isn't possible to update it currently.
> 
> A year or so ago I clean installed with a software RAID 1 array using
> /dev/sda & /dev/sdb and two partitions, md1 & md2 configure
> automatically on install.
> 
> I restored data to the RAID and then added manually added a third drive
> /dev/sdc as a spare.
> 
> All appeared hunky dory, but whilst trying to figure a slightly
> different problem on a different machine, I went back to the first one
> to check how it was configured. Although I am sure all looked normal
> when I had last looked, this time is looked a bit strange.
> 
> Unfortunately I don't have an exact copy of things before I started
> messing about but it looked something like this :
> 
> 
> cat /proc/mdstat revealed :
> 
> Personalities : [raid1]
> md1 : active raid1 sdc1[2] sdb1[1] sda1[0]
>       104320 blocks [3/3] [UUU]
> 
> md2 : active raid1 sdc2(S) sdb2[1] sda2[0]
>       244091520 blocks [2/2] [UU]
> 
> unused devices: <none>
> 
> 
> I don't understand how md1 shows [UUU] ??

You have a RAID1 with 3 devices.  What it difficult to comprehend about that.
Each block is written to all three devices.

You don't want that?  Change it.

   mdadm --grow /dev/md1 --raid-disks=4

now it has 4 devices - though one will be missing.

   mdadm --grow /dev/md1 --raid-disks=2

now it has 2 devices.  Actually that won't work until you mark one of the
devices as failed, so

   mdadm /dev/md1 --fail /dev/sdc1
   mdadm --grow /dev/md1 --raid-disks=2


But maybe you really wanted a 3-disk RAID1 before - it is a configuration
that certainly has a place.

Next you'll be telling me that a RAID5 cannot be made with just 2 devices!
:-)

NeilBrown


> 
> 
> On my other machine which has a similar configuration it shows the
> following which I expect :
> 
> Personalities : [raid1]
> md1 : active raid1 sda1[2](S) hdc1[1] hda1[0]
>       104320 blocks [2/2] [UU]
> 
> md2 : active raid1 sda2[2](S) hdc2[1] hda2[0]
>       312464128 blocks [2/2] [UU]
> 
> unused devices: <none>
> 
> I thought I could fail and remove the drive, dd/fdisk/reformat, sfdisk
> and then try to re add it back to the array effectively as a new drive.
> No joy.
> 
> If I just fail and remove it md1 shows as [UU_]
> 
> I have tried checking mdadm.conf which has the following :
> 
> DEVICE partitions
> ARRAY /dev/md1 level=raid1 num-devices=2
> uuid=8833ba3d:ca592541:20c7be04:42cbbdf1 spares=1
> ARRAY /dev/md2 level=raid1 num-devices=2
> uuid=43a5b70d:9733da5c:7dd8d970:1e476a26 spares=1
> 
> Somewhere along the line the RAID is remembering the earlier
> configuration but having changed stuff left right and Cambridge, I can't
> seem to get it to forget.
> 
> I have tried different variations of mdadm.conf, and tried to rebuild
> initrd but that didn't fix it and I am clean out of ideas where to go
> next. mdadm.conf seems to be ignored.
> 
> Undoubtedly it will take some clever tweaking and I'm scared witless at
> trashing the array as I am in a different country from the hardware and
> would struggle to get back to fix it !
> 
> Any advice on how to put it back to RAID 1 with a 'hot' spare would be
> appreciated.
> 
> B. Rgds
> John
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