Re: raid0 not growable?

[Neil Brown <neilb@suse.de>]

On Wed, 23 Dec 2009 23:28:37 +0000
Kristleifur Daðason <kristleifur@gmail.com> wrote:

> On Wed, Dec 23, 2009 at 10:45 PM, Neil Brown <neilb@suse.de> wrote:
> > On Wed, 23 Dec 2009 13:52:54 +0000
> > Kristleifur Daðason <kristleifur@gmail.com> wrote:
> >
> >> Hi,
> >>
> >> I'm running a raid0 array over a couple of raid6 arrays. I had planned
> >> on growing the arrays in time, and now turns out to be the time.
> >>
> >> To my chagrin, searches indicate that raid0 isn't growable. Can anyone
> >> confirm this before I wipe and reconfigure?
> >
> > That is correct, you cannot currently grow md/raid0.
> >
> > If the two raid6 arrays are exactly the same size, then you
> > could grow the two raid6 arrays, create a new raid0 over them
> > with the same block size and all your data will still be there.
> > But if they are not the same size, that won't work.
> >
> > NeilBrown
> >
> 
> Many thanks for the reply. (Re-cc'd to the linux-raid list. Hope that's OK.)

Certainly.  I didn't mean to drop linux-raid - I must have clicked the wrong
button.

> 
> 1.
> The raid6 arrays are exactly alike. Do I just do a create a new raid0
> with the right size, device count and chunk-size parameters? I trust
> your advice, but I am also certain of my own foolishness. I can't
> fully picture what happens to the data on the array -- Specifically,
> I'm thinking about whether to use --assume-clean or not. The
> documentation says not. I am guessing that a newly-created raid0
> doesn't do any syncing/resyncing anyway - it just sets up the array
> structure and metadata and I am left to my own devices to fill it with
> data.
> 
> Current chunksize is 256 and metadata is 1.1. So it's just a "mdadm
> --create /dev/md_bigraid0 --level=0 --raid-devices=2 --metadata=1.1
> --chunksize=256 /dev/md_raid6a /dev/md_raid6b", right?

Yes... there is a possible complication though.
With 1.1 metadata mdadm reserves some space between the end of the metadata
and the start of the data for a bitmap - even for raid0 which cannot have
a bitmap.  The amount of space reserved is affected by the size of the
devices.
So it is possible that the "data offset" will be different.
You should check the data offset before and after.  If it is different, we
will have to hack mdadm to allow you to set the data offset manually.

> 
> 2.
> I have a JFS filesystem on the big raid0. Once I have the bigger raid0
> built, I assume that I would first do a read-only fsck.jfs, which will
> succeed if I did everything correctly. Then I do a remount with the
> "resize" option to JFS to finally grow the JFS filesystem.

Sounds right (though I've never used jfs).

> 
> --
> 
> Sincere thanks. I hope I shall be able to contribute something
> meaningful to mdadm when the company is richer and my time is freer :)
> In the meantime, is there any preferred way of donating to mdadm or
> sponsoring it?
>
Just use it and report any issues you have, as you have done.

NeilBrown
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