Richard Scobie wrote:
Have a look at the "Grow Mode" section of the mdadm man page.

Thanks! I overlooked that, although I did look at the man page before posting.

It looks as though you should just need to use the same command you used to grow it to 3 drives, except specify only 2 this time.

I think I hot-added it. Anyway, --grow looks like what I need, but I'm having some difficulty with it. The man page says, "Change the size or shape of an active array." But I got:

[EMAIL PROTECTED] ~]# mdadm --grow /dev/md5 -n2
mdadm: Cannot set device size/shape for /dev/md5: Device or resource busy
[EMAIL PROTECTED] ~]# umount /dev/md5
[EMAIL PROTECTED] ~]# mdadm --grow /dev/md5 -n2
mdadm: Cannot set device size/shape for /dev/md5: Device or resource busy

So I tried stopping it, but got:

[EMAIL PROTECTED] ~]# mdadm --stop /dev/md5
[EMAIL PROTECTED] ~]# mdadm --grow /dev/md5 -n2
mdadm: Cannot get array information for /dev/md5: No such device
[EMAIL PROTECTED] ~]# mdadm --query /dev/md5 --scan
/dev/md5: is an md device which is not active
/dev/md5: is too small to be an md component.
[EMAIL PROTECTED] ~]# mdadm --grow /dev/md5 --scan -n2
mdadm: option s not valid in grow mode

Am I trying the right thing, but running into some limitation of my version of mdadm or the kernel? Or am I overlooking something fundamental yet again? md5 looked like this in /proc/mdstat before I stopped it:

md5 : active raid1 hdc8[2] hdg8[1]
     58604992 blocks [3/2] [_UU]

For -n the man page says, "This number can only be changed using --grow for RAID1 arrays, and only on kernels which provide necessary support."

Grow mode says, "Various types of growth may be added during 2.6 development, possibly including restructuring a raid5 array to have more active devices. Currently the only support available is to change the "size" attribute for arrays with redundancy, and the raid-disks attribute of RAID1 arrays. ... When reducing the number of devices in a RAID1 array, the slots which are to be removed from the array must already be vacant. That is, the devices that which were in those slots must be failed and removed."

I don't know how I overlooked all that the first time, but I can't see what I'm overlooking now.

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

Cheers,
11011011
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