RE: How to un-degrade an array after a totally spurious failure?

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-----Original Message-----
From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
owner@vger.kernel.org] On Behalf Of NeilBrown
Sent: Wednesday, May 20, 2009 9:49 PM
To: Nix
Cc: linux-raid@vger.kernel.org
Subject: Re: How to un-degrade an array after a totally spurious
failure?

On Thu, May 21, 2009 9:10 am, Nix wrote:

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So, anyone got a command that would help? I'm not even sure if this

is

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assembly or growth: it doesn't quite fit into either of those
categories. There must be a way to do this, surely?

It is neither.  It is management.

 mdadm --manage /dev/mdX --remove /dev/sdb6
 mdadm --manage /dev/mdX --add /dev/sdb6

(The --manage is not actually needed, but it doesn't hurt).

NeilBrown

	I have exactly the same situation, except there are two "failed"
disks on a RAID5 array.  As for the OP, the "failures" are spurious.
Running the remove and then the add command puts the disks back in as
spare
disks, not live ones, and then the array just sits there, doing nothing.
I
tried the trick of doing

echo repair > /sys/block/md0/md/sync_action

but the array still just sits there saying it is "clean, degraded", with

2

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spare and 5 working devices.

No such a good option when you have two failures.
If you have two failures you need to stop the array, then assemble
it again using --force.
It is now too late for that:  adding them with "--add" will have erased
the old metadata.

OK, what about a RAID 6 array with one failed disk?  Is the remove & add the
best option there, or should one apply a different method?