Re: raid0 not growable?

["Kristleifur Daðason" <kristleifur@gmail.com>]

On Wed, Dec 30, 2009 at 3:25 PM, Kristleifur Daðason
<kristleifur@gmail.com> wrote:
> On Wed, Dec 23, 2009 at 11:54 PM, Neil Brown <neilb@suse.de> wrote:
>> 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.
>>> >>
>>> > 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.
>>>
>>> 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.
>
> ... I believe I am guaranteed an identical bitmap size and hence an identical data offset.
>
> And in theory, this case is closed. Thank you, all.
>

Yep, it worked great. We built the new raid0 array over the old one,
and did a "fsck.jfs -n" dry-run over the filesystem. Still there,
clean as a whistle. Next was a quick "mount -o remount,resize /tank"
which grew the JFS filesystem in a second or two.

Very quick and painless. For my purposes, I consider raid0 arrays to
be growable. Such was the ease. Even though raid0 may not be
officially growable.

-- Kristleifur
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