More Zonkyness

Whether you are using ufs filesystems or zfs storage pools, Solaris has a rather nifty way of migrating storage from one SAN to another wih no (or little) downtime. Or various other reasons involving moving from one disk to another. The key advantage to the following method is to reducing or eliminating downtime. Even if your users can take the hit, not having to slowly watch a multiterabyte filesystem copying from one disk to another is reason enough to use this technique.

Basically it is by using mirroring. Using mirroring to copy a disk might seem a little odd to begin with, but once you’ve seen it work you’ll be a fan.

For UFS (and SVM) Filesystems

This section assumes that the source disk device (cXXXXX) is set in the variable ${sourcedisk} and the destination is in ${destdisk}.

For UFS filesystems, the first step (which does require an outage) is to :-

1. Stop the application that uses the filesystem being migrated.
2. Unmount the filesystem.
3. Encapsulate the existing filesystem device into a SVM metadevice: metainit d1001 1 1 ${sourcedisk}
4. Create a mirror device with the new metadevice as a submirror: metainit d1000 -m d1001
5. Change the references in /etc/vfstab to the old device name (${sourcedisk}) to the new mirror (not sub-mirror!) device – d1000
6. Remount the filesystem and restart the application.

This should take no more than 10 minutes and is the only outage involved. There are two remaining sets of steps :-

1. Create a new metadevice using the new disk: metainit d1002 1 1 ${destdisk}
2. Attach the new metadevice to the mirror as an additional sub-mirror: metattach d1000 d1002

At this point, the mirror will start resilvering. It may take some time to complete, but the time it takes to do so does not really matter. In particular the resilvering process should not cause a performance problem to your application – the application I/O takes priority.

When the resilvering is complete :-

1. Remove the metadevice containing the old SAN disk: metadetach d1000 d1001
2. Remove the metadevice that is no longer required: metaclear d1001
3. Attach “nothing” to the mirror metadevice (this is to ensure that the mirror grows to the size of the new submirror): metattach d1000
4. Finally, ignore the warning on the manual page (which is outdated) and grow the filesystem: growfs -M /mount/point /dev/md/rdsk/d1000

You will see that I have used the metadevice names d1000 (for the mirror), d1001 (for the old sub-mirror), and d1002 (for the new submirror). Whatever device names you use, it is worth trying to be consistent – it helps a lot when you have dozens of filesystems to process.

ZFS Storage Pools

This is even simpler. If you have a storage pool called ${pool} which contains a single device called ${sourcedisk}, you simply :-

1. Attach the new device: zpool attach ${pool} ${sourcedisk} ${destdisk}
2. Wait for the resilvering to finish.
3. Dettach the old device: zpool detach ${pool} ${sourcedisk}

Of course be aware of anything you read on the Internet! I have not actually tested the above; I’m merely regurgitating memory that has recently been exercised – I’m doing a SAN migration at work right now.