On Tue, 14 Nov 2006, Will Sheffler wrote:

Hi.

What is the safest way to switch out a disk in a software raid array created
with mdadm? I'm not talking about replacing a failed disk, I want to take a
healthy disk in the array and swap it for another physical disk. Specifically,
I have an array made up of 10 250gb software-raid partitions on 8 300gb disks
and 2 250gb disks, plus a hot spare. I want to switch the 250s to new 300gb
disks so everything matches. Is there a way to do this without risking a
rebuild? I can't back everything up, so I want to be as risk-free as possible.

I guess what I want is to do something like this:

(1) Unmount the array
(2) Un-create the array
(3) Somehow exactly duplicate partition X to a partition Y on a new disk
(4) Re-create array with X gone and Y in it's place
(5) Check if the array is OK without changing/activating it
(6) If there is a problem, switch from Y back to X and have it as though
nothing changed

The part I'm worried about is (3), as I've tried duplicating partition images
before and it never works right. Is there a way to do this with mdadm?

if you have a recent enough kernel (2.6.15 i think) and recent enough 
mdadm (2.2.x i think) you can do this all online without losing redundancy 
for more than a few seconds... i placed a copy of instructions and further 
discussions of what types of problems this method has here:

http://arctic.org/~dean/proactive-raid5-disk-replacement.txt

it's actually perfect for your situation.

-dean