Re: Can't expand linear RAID on top of 2 x RAID1

[NeilBrown <neilb@suse.de>]

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On Sat, 23 Jun 2012 12:53:12 +1000 Adam Goryachev
<mailinglists@websitemanagers.com.au> wrote:

> Stan Hoeppner <stan@hardwarefreak.com> wrote:
> 
> >On 6/22/2012 5:41 AM, Adam Goryachev wrote:
> >> I have expanded my system over time, I started with 2 x 2TB drives in
> >> RAID1 (md2)
> >> 
> >> I then added 2 x 750GB drives, configured as RAID1 (md1)
> >> Then created a third raid (md3) as linear with the md2 + md1
> >> 
> >> Finally, I've upgraded the 2 x 750G to 2 x 1TB drives (one at a
> >time).
> >> 
> >> I then did a mdadm --grow to expand the RAID1 from 750G to 1TB
> >> 
> >> The problem I am having is that I can't expand the linear (md3) array
> >to
> >> grow the extra 250G of space.
> >> 
> >> Could anyone suggest how I might be able to get the extra 250G of
> >space
> >> to become available?
> >
> >If you think about this for a few minutes more, and re-read how md
> >--linear works, and thus how growing a linear array works, you'll
> >surely
> >understand why you can't do what you're attempting to do.
> 
> Actually, no I don't understand the problem... From my (probably limited) understanding, linear simply appends each drive to the array. I understand that you would not be able to increase the size of any constituent device which is not the last component of the array, but I don't see any reason why it is not possible to increase the size of the last component. (Which is what I am attempting to do)
> 
> >As for seeing that extra 250GB, I don't have an answer.  Typically
> >linear arrays are used in lieu of growing constituent member arrays.
> >That's kinda the whole point of linear (concatenation).
> 
> Well, the only other alternative I can think of is to reduce the size of the array back to 750G, reduce the size of the partitions for that array to 750G. Then create a new 250G partition on the two 1TB drives, create a fourth RAID1 array, and then add that 4th RAID1 array to the end of the existing linear array. This just didn't seem like a very good solution (ie, not clean).
> 
> >You could try deleting the linear array and simply creating a new one.
> >But surely the changed offsets would wreak havoc on the filesystem
> >currently spanning this mess.
> 
> Perhaps this would work. If I delete the linear array and re-create, with assume-clean (or whatever the right flag is, I'll read the man page later when I'm doing it), then like I said, the first component device will be the same, the second component device is the same, but just happens to be bigger. Any comments on whether this should work?

The is no '--assume-clean' for linear.  Linear arrays cannot be clean or
dirty as md stores no redundant information.

Just re-creating the array would probably work, but there is an easier way.

md doesn't record the size of the array for Linear and RAID0.  It just adds
together the sizes of the devices that it finds.

With 1.x metadata, md does record the effective size of each devices, so adds
those together.
You can update this effective size by assembling with 
   --update=devicesize

I just checked this will loop devices and it works as expected (assuming you
have 1.1 or 1.2 metadata).
So:
  mdadm -S /dev/md2
  mdadm -A --update=devicesize /dev/md0 /dev/md1

(order doesn't matter with assemble).

> 
> What should I do to ensure that the component devices are ordered the way I think they are? Apparently the numbers in /proc/mdstat just tell me what order the devices were added, not what order they are in the array?

mdadm --detail  or mdadm --examine would tell you what you want to know.

NeilBrown


> 
> Thank you for your advice.
> 
> Regards,
> Adam
> 
> 
> 
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