From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.de>
Subject: Re: Backup file size when migrating from raid5 to raid6?
Date: Sun, 6 May 2012 21:00:28 +1000
Message-ID: <20120506210028.6212b2f8@notabene.brown>
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To: =?ISO-8859-1?Q?Gar=F0ar?= Arnarsson <gardar@giraffi.net>
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On Sun, 6 May 2012 10:17:52 +0000 Gar=C3=B0ar Arnarsson <gardar@giraffi.net=
> wrote:

> My raid5 array has gotten a bit big, it's containing total 10 drives
> right now (I started out with 3 drives). So I am going to convert it
> to raid6 before it gets any bigger.
>=20
> I am doing a test-run on a virtual machine with virtual drives to see
> that everything works flawlessly.
>=20
> When I tried to convert the array to raid6 I got a error message about
> a missing backup-file
>=20
> mdadm --grow /dev/md0 --raid-devices=3D5 --level=3D6
>=20
> mdadm level of /dev/md0 changed to raid6
> mdadm: /dev/md0: Cannot grow - need backup-file
> mdadm: aborting level change
>=20
> I added the backup file and was able to convert the array successfully
> after that.
>=20
> My question is, how big is this backup file going to be? My real raid
> array consists of 2tb drives, will the backup file be as big as one
> drive in the array, or will it just be few megabytes or gigabytes?
> I'm asking because I'm wondering if I need to buy an extra hdd for the
> backup file or if the backup file can just be on my OS hdd that has
> around 100gb free.

The backup file is a few megabytes. Around 16MB I think.

However if you are likely to add another device in the not too distant futu=
re
you can save yourself a bit of time.

If you

 mdadm --grow /dev/md0 --level=3D6 --layout=3Dpreserve

It will just make the new few a 'Q-block' device, containing the extra RAID6
'parity' block for each stripe.  This doesn't require any reshape or or any
backup file and is a lot faster.  All it requires is a normal recovery
operation.

Then when you later add another device you can

  mdadm --grow /dev/md0 --raid-devices=3DN+1 --layout=3Dnormalise

This will convert from the Q-on-the-last-device layout to a more normal
rotated-P-and-Q layout at the same time as adding extra space.

NeilBrown

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