Re: Backup file size when migrating from raid5 to raid6?

[NeilBrown <neilb@suse.de>]

[-- Attachment #1: Type: text/plain, Size: 3088 bytes --]

On Mon, 7 May 2012 00:32:35 +0000 Garðar Arnarsson <gardar@giraffi.net> wrote:

> That's an excellent idea, I was going to add another disk for extra space
> right after migrating to raid6.
> 
> Just to be clear, I'll be running the normalize attribute just once to
> straighten the array out right? Or will I have to do it for every extra
> drive I add in the future?

Just once.

> 
> And what are the N+1 you mention in --raid-devices=N+1

By "N+1" I just meant "1 more than the number of devices currently in the
array".

If you have both new devices ready to go, you just do a single reshape
operation that converts to RAID6 and adds more space.  This does not need a
backup file and is probably the best approach.

If you currently have a 10-drive RAID5 and want a 12-drive RAID6, then

 mdadm --grow /dev/md0 --raid-devices=12 --level=6

is what you want.

NeilBrown


> 
> Thanks.
> 
> 
> 2012/5/6 NeilBrown <neilb@suse.de>
> 
> > On Sun, 6 May 2012 10:17:52 +0000 Garðar 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.
> > >
> > > I am doing a test-run on a virtual machine with virtual drives to see
> > > that everything works flawlessly.
> > >
> > > When I tried to convert the array to raid6 I got a error message about
> > > a missing backup-file
> > >
> > > mdadm --grow /dev/md0 --raid-devices=5 --level=6
> > >
> > > mdadm level of /dev/md0 changed to raid6
> > > mdadm: /dev/md0: Cannot grow - need backup-file
> > > mdadm: aborting level change
> > >
> > > I added the backup file and was able to convert the array successfully
> > > after that.
> > >
> > > 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
> > future
> > you can save yourself a bit of time.
> >
> > If you
> >
> >  mdadm --grow /dev/md0 --level=6 --layout=preserve
> >
> > 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=N+1 --layout=normalise
> >
> > 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
> >
> 
> 
> 


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 828 bytes --]