Re: raid0 not growable?

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

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.


Thank you for the replies, Neil and everybody.

As we rise from Christmas, bloated to satisfaction, we are in spirits
to grow the RAID. Following your information about the bitmap size and
the data offset, I had a quick dig through the mdadm 3.1.1 source [1].

In "super1.c:static unsigned long choose_bm_space(unsigned long
devsize)", it says "if the device is bigger than 8Gig, save 64k for
bitmap usage, if bigger than 200Gig, save 128k".

In this case, I have two raid6 devices under the raid0. I have grown
the raid6 devices from ~3TB to ~6TB each. Unless I am mistaken, the
devices are far bigger than the threshold to the 128k bitmap size,
both before and after growth. Hence, 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.

-- Kristleifur


[1] Turns out that Textmate on OS X is a very nice tool for studying
open sourcecode. The times I have dug through source before, I've
usually gotten lost in the trees. Textmate was refreshing - it felt
like the source was levitating in my hand, elegantly twisting and
turned and revealing itself. Textmate is just an editor of course, but
it's comfortable to the point of being magical.
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