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

[NeilBrown <neilb@suse.de>]

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On Thu, 16 Aug 2012 17:28:13 -0600 Thomas Fjellstrom <thomas@fjellstrom.ca>
wrote:

> 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.
> 

If you change the level and the number of devices in the one command, and
there are already enough spares present to satisfy the requirements of the
new size, then it will only perform a single reshape operation which will use
the minimal amount of time.

NeilBrown


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