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On 18/04/12 01:12, NeilBrown wrote:

On Wed, 18 Apr 2012 00:42:54 +0200 John Crisp wrote:

quoted

I don't understand how md1 shows [UUU] ??

You have a RAID1 with 3 devices.  What it difficult to comprehend
about that.

Probably a misunderstanding of the terminology - in Hicksville where I
live mirrored usually just means a pair ;-)

Each block is written to all three devices.

I didn't realise that a RAID 1 could have more than two devices. Most
documentation around always illustrates a RAID 1 with a pair of drives
and hence the confusion.

You don't want that?  Change it.

mdadm --grow /dev/md1 --raid-disks=4

now it has 4 devices - though one will be missing.

Logical if you know it (RAID 1) can have more devices.

mdadm --grow /dev/md1 --raid-disks=2

Illogical naming if what you actually want to do is shrink the
array............ like many things in life, once you know, you know.

now it has 2 devices.  Actually that won't work until you mark one
of the devices as failed, so

mdadm /dev/md1 --fail /dev/sdc1 mdadm --grow /dev/md1
--raid-disks=2

And there lies the answer to my problem. For that I can't thank you
enough !

But maybe you really wanted a 3-disk RAID1 before - it is a
configuration that certainly has a place.

No I didn't, but I now understand.

Next you'll be telling me that a RAID5 cannot be made with just 2
devices! :-)

Me ? I know jack ...... though I know one more thing than I did
yesterday, and that is good enough for me !

Many thanks for the assistance.

B. Rgds
John
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