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

[Thomas Fjellstrom <thomas@fjellstrom.ca>]

On Sun May 6, 2012, NeilBrown wrote:
> 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.

I apologize for bringing back a long dead thread, but I've been wondering if 
mdadm does the grow op in this case, in one step? Or does it internally do 
each step separately, doing a reshape with each one?

I've currently got a 7x1TB disk raid5, and have a couple more disks to add and 
I was planning on moving to raid6. I'm hoping to reduce the amount of time the 
array is "reshaping" because I'm a bit paranoid that my bad luck with hard 
drives will decide to hit right then and there.

> 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
> > >
> > 
> > 
> > 
> 
> 


-- 
Thomas Fjellstrom
thomas@fjellstrom.ca
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