On 07/05/20 20:36, David F wrote:

I'm planning to use use three mdadm commands to accomplish the
reshape, as follows:

mdadm --grow --size=6000G --assume-clean /dev/md0
mdadm --grow --array-size=12000G /dev/md0
mdadm --grow --raid-devices=3 --backup-file=/root/md-backup /dev/md0


Ideally, I'd prefer a single command,
mdadm --grow --size=6000G --raid-devices=3 /dev/md0
... but that seems not possible [1].


In either case, my question still applies: when reshaping to
reduce the number of devices in the array (--raid-devices), is
there any way to specify exactly which device(s) are to be
removed from active sync (I suppose they become spares) and which
ones kept? 

You should be able to stick --remove /dev/sdX on the command line ...

BUT. Before you start, what version of mdadm are you running? The
latest? What version of linux? An old LTS with a heavily patched
frankenkernel? You really want to be running both a recent kernel and
recent mdadm.

The other point, if you don't need --backup, DON'T USE IT. It doesn't do
any real harm provided you use it right, but it shouldn't be necessary
and it causes needless confusion and trouble.

Cheers,
Wol