Shrinking a RAID1 device mounted as root?

Shrinking a RAID1 device mounted as root?

[Marlier, Ian]

Hey all...

I'm looking for some help figuring out how to shrink a RAID1 device
that's mounted as root on a Red Hat system.

The device setup is this:
Raiddev	/dev/md0
	Device	/dev/hda1
	Raid-disk	0
	Device	/dev/hdb1
	Raid-disk	1

Hda and hdb are both 36GB ATA hard drives, and each currently has a
partition of about 33GB formatted as Linux RAID autodetect.  Each also
has a swap partition.

What I'd like to do is shrink partition 1 on each disk to around 16GB,
and then create a second RAID1, /dev/md1, with a new partition /dev/hda3
and /dev/hdb3 (leaving the swap device alone.)

I booted off a rescue disk with mdadm, resize2fs, fdisk, etc, and I was
able to shrink /dev/md0 down to 16GB.

I then tried to delete /dev/hda1 and /dev/hdb1 using fdisk, and create 2
new equally-sized partitions in the free space created.  (/dev/hda1,
/dev/hda3, /dev/hdb1, /dev/hdb3, all type Linux RAID Autodetect).
Because /dev/hd[ab]1 is larger than /dev/md0, and the starting block of
the partitions is the same, the filesystem should remain intact.

However, upon rebooting, I ran into a problem.  /dev/md0 cannot be
mounted because of a problem with the superblocks.

I figure that something needs to be done with mdadm to get it to
recognize the "new" /dev/hda1 and /dev/hdb1 as /dev/md0 -- but I don't
want to do a mdadm --create because that will overwrite the filesystem
already on the disks (I think?). 

Is this something that mdadm --assemble can do?  If so, what switches
need to be fed to the command?

Other suggestions?

Re: Shrinking a RAID1 device mounted as root?

[Mike Tran]

On Wed, 2004-07-14 at 10:47, Marlier, Ian wrote:
> Hey all...
> 
> I'm looking for some help figuring out how to shrink a RAID1 device
> that's mounted as root on a Red Hat system.
> 
> The device setup is this:
> Raiddev	/dev/md0
> 	Device	/dev/hda1
> 	Raid-disk	0
> 	Device	/dev/hdb1
> 	Raid-disk	1
> 
> Hda and hdb are both 36GB ATA hard drives, and each currently has a
> partition of about 33GB formatted as Linux RAID autodetect.  Each also
> has a swap partition.
> 
> What I'd like to do is shrink partition 1 on each disk to around 16GB,
> and then create a second RAID1, /dev/md1, with a new partition /dev/hda3
> and /dev/hdb3 (leaving the swap device alone.)
> 
> I booted off a rescue disk with mdadm, resize2fs, fdisk, etc, and I was
> able to shrink /dev/md0 down to 16GB.
> 
> I then tried to delete /dev/hda1 and /dev/hdb1 using fdisk, and create 2
> new equally-sized partitions in the free space created.  (/dev/hda1,
> /dev/hda3, /dev/hdb1, /dev/hdb3, all type Linux RAID Autodetect).
> Because /dev/hd[ab]1 is larger than /dev/md0, and the starting block of
> the partitions is the same, the filesystem should remain intact.
> 
> However, upon rebooting, I ran into a problem.  /dev/md0 cannot be
> mounted because of a problem with the superblocks.
> 
> I figure that something needs to be done with mdadm to get it to
> recognize the "new" /dev/hda1 and /dev/hdb1 as /dev/md0 -- but I don't
> want to do a mdadm --create because that will overwrite the filesystem
> already on the disks (I think?). 
> 
> Is this something that mdadm --assemble can do?  If so, what switches
> need to be fed to the command?
> 
> Other suggestions?
> 
> 

If your newly created partitions are > 16GB as stated, you should be
able to recreate raid1 md0 using mdadm via your rescue disk.  After the
re-sync completes, you can reboot the machine.  Remember, raid1 is
mirroring :)

Regards,
Mike T.

Re: Shrinking a RAID1 device mounted as root?

[Paul Clements]

Marlier, Ian wrote:

> Because /dev/hd[ab]1 is larger than /dev/md0, and the starting block of
> the partitions is the same, the filesystem should remain intact.

So you are saying that the filesystem was less than 16GB in size to 
start with?

> However, upon rebooting, I ran into a problem.  /dev/md0 cannot be
> mounted because of a problem with the superblocks.

Right, the superblocks would have been between 64KB and 128KB from the 
end of the original md device, which presumably was ~33GB in size. Now 
that the partitions are smaller, the superblocks are no longer located 
inside of the partitions, so the new partitions effectively have no 
superblocks.

> I figure that something needs to be done with mdadm to get it to
> recognize the "new" /dev/hda1 and /dev/hdb1 as /dev/md0 -- but I don't
> want to do a mdadm --create because that will overwrite the filesystem
> already on the disks (I think?). 

Create will write new superblocks to /dev/hda1 and /dev/hdb1. Without 
these, the array cannot be started. The superblocks are what tell the md 
driver that /dev/md0 is composed of /dev/hda1 and /dev/hdb1.

If the filesystem is at least 128KB smaller than the partitions, then 
you'll have room for the superblocks at the end. Otherwise, go back with 
fdisk and make the partitions just a little bit larger and then do your 
mdadm --create.


> Is this something that mdadm --assemble can do?  If so, what switches
> need to be fed to the command?

Assemble requires that the superblocks already be present, so until they 
are re-created, you cannot assemble the array.

--
Paul

RE: Shrinking a RAID1 device mounted as root?

[Guy]

The superblock is at the end of the partition.  If the size is different, md
will not find the superblock.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Marlier, Ian
Sent: Wednesday, July 14, 2004 11:48 AM
To: linux-raid@vger.kernel.org
Subject: Shrinking a RAID1 device mounted as root?

Hey all...

I'm looking for some help figuring out how to shrink a RAID1 device
that's mounted as root on a Red Hat system.

The device setup is this:
Raiddev	/dev/md0
	Device	/dev/hda1
	Raid-disk	0
	Device	/dev/hdb1
	Raid-disk	1

Hda and hdb are both 36GB ATA hard drives, and each currently has a
partition of about 33GB formatted as Linux RAID autodetect.  Each also
has a swap partition.

What I'd like to do is shrink partition 1 on each disk to around 16GB,
and then create a second RAID1, /dev/md1, with a new partition /dev/hda3
and /dev/hdb3 (leaving the swap device alone.)

I booted off a rescue disk with mdadm, resize2fs, fdisk, etc, and I was
able to shrink /dev/md0 down to 16GB.

I then tried to delete /dev/hda1 and /dev/hdb1 using fdisk, and create 2
new equally-sized partitions in the free space created.  (/dev/hda1,
/dev/hda3, /dev/hdb1, /dev/hdb3, all type Linux RAID Autodetect).
Because /dev/hd[ab]1 is larger than /dev/md0, and the starting block of
the partitions is the same, the filesystem should remain intact.

However, upon rebooting, I ran into a problem.  /dev/md0 cannot be
mounted because of a problem with the superblocks.

I figure that something needs to be done with mdadm to get it to
recognize the "new" /dev/hda1 and /dev/hdb1 as /dev/md0 -- but I don't
want to do a mdadm --create because that will overwrite the filesystem
already on the disks (I think?). 

Is this something that mdadm --assemble can do?  If so, what switches
need to be fed to the command?

Other suggestions?



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