How to resize RAID-1 partitions (mdraid)

[Sam Varshavchik]

Roberto Ragusa writes:

> I never used parted, so if I had to achieve what you want I'd try something
> completely different.

You have to use something to adjust the size of the ext3 filesystem. Growing 
the underlying mdraid device is not sufficient. Once the underlying block 
device is larger, the ext3 filesystem needs to be grown, else it'll remain 
at its logical size, with no benefit.

I don't think that mdadm --grow adjusts the ext3 metadata on the partition 
to reflect its new size. Anything can be on a RAID block device, not just 
ext3. Something still needs to logically resize the ext3 filesystem.

> Boot with some sort of rescue disk so you are not running from the disks.
> 
> Break the RAID eliminating the sdb partitions to be modified:
> (md1 and md2 is my guess at the names)
> 
> mdadm /dev/md1 --fail /dev/sdb1
> mdadm /dev/md2 --fail /dev/sdb2
> mdadm /dev/md1 --remove /dev/sdb1
> mdadm /dev/md2 --remove /dev/sdb2
> mdadm /dev/md1 --grow -n 1
> mdadm /dev/md2 --grow -n 1

Why do you need to --grow here?

> So you now have md1 and md2 are a 1-disk RAID1, while md5 is still running
> on two disks.
> 
> You now fdisk /dev/sdb: delete all the partitions and immediately recreate
> them with the new sizes. Be sure that sdb4 and sdb5 are recreated with _exactly_
> the same positions they had before. Do not use "fd" as type for sdb1 and sdb2,

Why not?

> for now. Now save the changes. The kernel will refuse to load the new partitions
> in memory as your md5 is still using sda5 and sdb5.
> 
> So you reboot.
> 
> Now you have again md1 (degraded) md2 (degraded) and md5 (2 disks!) available.
> Create two new degraded RAID devices:
> 
> mdadm --create /dev/md11 -l 1 -n 1 /dev/sdb1
> mdadm --create /dev/md12 -l 1 -n 1 /dev/sdb2
> 
> (mdadm will also want "-f" because n=1 is unusal)
> 
> Now you can mkfs /dev/md11 and /dev/md12 and cp from /dev/md1 and /dev/md2

I see what you're getting at here. I'm wondering whether it's even necessary 
to create temporary md devices and manually copy over. Can't I just create 
the partitions on the second drive, in their new position, add them to my 
existing RAID sets, and let the kernel sync up the data, then break the RAID 
volumes again, rebuild the partitions on the first drives, add them back 
into the RAID, sync the data back up to the primary, "--grow max", now that 
the first partition is larger on both drives, to have mdraid grow the RAID 
volume to its new bigger size, then finally parted to grow the ext3 
filesystem on the larger RAID volume.

Then I have another server that does not have a luxury of a larger swap 
partition that can be conveniently shrunk. But I think I can see how that 
can be done using documented parted abilities: shrink the large ext3 
partition using parted. Shrink its RAID device using mdadm --grow. Degrade 
the RAID by taking out the partition from one of the drives. Delete the 
physical partition and recreate it its new position. Add it back into the 
RAID volume, let the kernel sync it, then repeat the same process with the 
partition on the other drive.

Now, I just need to understand why you say not to use the fd partition type.


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