You should never degrade the array to copy its contents. Its very
risky. There might be bad sectors on one or more disks and if you
don't have all disks at hand, you may not be able to rebuild the array
on the new disk.

As for using dd, as others have pointed, the metadata won't be in
place and if you create a new array after using dd, it'll still
require a resync and the data will be destroyed.

On Sat, Mar 13, 2010 at 6:18 PM, John Robinson
[off-list ref] wrote:

On 12/03/2010 18:09, Luca Berra wrote:

quoted

On Fri, Mar 12, 2010 at 05:39:47PM +0000, John Robinson wrote:

quoted

Before I have to go to a customer's premises and try it, I wanted to ask
if anyone had already done anything like this: I have a CentOS 5 system with
0.90 metadata and type fd partitions with RAID-5 over 3 discs. I want to
grow it by replacing the drives. If I dd the small partitions onto larger
ones on the new discs, then reboot, will the system recognise the RAID-5 -
which now has its metadata somewhere in the middle of the paritions not at
the end - so that I can then --grow?

I believe it won't work
mdadm has no way to know where the metadata is in the above case.

you have to follow the painful procedure
fail one drive (mdadm /dev/mdX --fail /dev/sdY)
remove it (mdadm /dev/mdX --remove /dev/sdY)
replace drive with larger one
create larger partition
add drive again (mdadm /dev/mdX --add /dev/sdY)
wait for it to resync...
...and repeat until all drives have been replaced

Yeuch. That'll take forever, because I'd need to rebuild 3 times. I see two
other options:
(1) dd the drives as I said originally, and when the system can't find the
array on the bigger discs, recreate the array using the same options the
original array had
(2) degrade the existing array to two discs, replace the soft-failed disc
with one of the new discs, temporarily make a filesystem on it, copy the
data; replace the other two discs, make a new array on them with a missing
drive, copy the data back; scrub the temporarily-made filesystem and add the
drive to the array.

Actually I think something along the lines of Majed B's suggestion - in this
case hang a spare drive out the side of the chassis, and just copy twice -
would be quicker than (2). But (1) should work, shouldn't it?

Cheers,

John.
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