Re: Shrinking an array

From: NeilBrown <neilb@suse.com>
To: Wakko Warner <wakko@animx.eu.org>, linux-raid@vger.kernel.org
Subject: Re: Shrinking an array
Date: Tue, 11 Apr 2017 14:33:24 +1000	[thread overview]
Message-ID: <87fuhfwokr.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <20170411003008.GA18538@animx.eu.org>

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On Mon, Apr 10 2017, Wakko Warner wrote:

> I have a question about shrinking an array.  My current array is 4x 2tb
> disks in raid6 (md0).  The array was created on the 2nd partition of each
> disk and spans most of the disk.  I would like to replace the 2tb disks with
> 750gb disks.  md0 is a luks container with lvm underneath.  I have less than
> 1tb actually in use.  What would the recommended procedure be for shrinking
> this?  I've watched this list, but I don't think I've come across anyone
> actually wanting to do this before.
> I'm thinking of these steps already:
> 1) Shrink PV.
> 2) Shrink luks.  I'm aware that there is not size metadata, but the dm
> mapping would need to be shrunk.
> 3) Shrink md0.  I did this once when I changed a 6 drive raid6 into a 5
> drive raid6.  Would I use --array-size= or --size= ?  I understand the
> difference is the size of md0 vs the individual members.

You don't need --array-size, as reducing --size is non-destructive.
Reducing the number of devices *is* destructive, so if you were to do
that, you would need to adjust the --array-size first.

So when you have prepared the contents of the array, use
  mdadm /dev/mdXXX --grow --size=750G

or whatever size you think is appropriate.

Then check that all your data is still accessible.  e.g. fsck your
filesystem.
If you are confident that the data is still accessible, you can continue
to --replace devices.
If not, you can
  mdadm /dev/mdXXX --grow --size=max

to restore the previous state, and then try to figure out what went
wrong.

NeilBrown

>
> So for number 4, if md0 is now small enough, will it accept a member that is
> smaller?  If so, I should beable to add the member to the array and issue
> --replace.
>
> Thanks.
>
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