Re: reducing the number of disks a RAID1 expects

[Neil Brown <neilb@suse.de>]

On Monday September 10, iusty@k1024.org wrote:
> On Sun, Sep 09, 2007 at 09:31:54PM -1000, J. David Beutel wrote:
> > [root@samue ~]# mdadm --grow /dev/md5 -n2
> > mdadm: Cannot set device size/shape for /dev/md5: Device or resource busy
> >
> > mdadm - v1.6.0 - 4 June 2004
> > Linux 2.6.12-1.1381_FC3 #1 Fri Oct 21 03:46:55 EDT 2005 i686 athlon i386 
> > GNU/Linux
> 
> I'm not sure that such an old kernel supports reshaping an array. The
> mdadm version should not be a problem, as that message is probably
> generated by the kernel.

2.6.12 does support reducing the number of drives in a raid1, but it
    will only remove drives from the end of the list. e.g. if the
    state was

      58604992 blocks [3/2] [UU_]

then it would work.  But as it is

      58604992 blocks [3/2] [_UU]

it won't.  You could fail the last drive (hdc8) and then add it back
in again.  This would move it to the first slot, but it would cause a
full resync which is a bit of a waste.

Since commit 6ea9c07c6c6d1c14d9757dd8470dc4c85bbe9f28 (about
2.6.13-rc4) raid1 will repack the devices to the start of the
list when trying to change the number of devices.

> 
> I'd recommend trying to boot with a newer kernel, even if only for the
> duration of the reshape.
> 

Yes, a kernel upgrade would do the trick.

NeilBrown