How to add disks to VGs in a running cluster

[Chris Bishop]

Here is my scenario. I have a two node cluster using shared disks from a Hitachi disk frame. This frame is older and is starting to fail. My storage admin has given me replacement LUNs from a new disk frame in order to mirror the data over and then remove the old disks. Another option would be to pvmove the data to the new disks and then remove the old disks. Is there any way to do this in a running cluster without taking it down?

Either way, what are the steps to safely get this done?

[Stephen Doud]

If the VG is activated in exclusive mode (check the 'Status' line in vgdisplay), then work this as a normal LVM admin task initially. If it is activated in shared mode, you have more steps to do than those below.

1. After you have added the new PVs to the VG, moved the data and removed the defective drives from the VG (pvremove) create a map file of the VG using:
# vgexport -ps -m .map /dev/
2.Copy it to the other node

Then on the other node...
3. Determine the VG's group minor number:
# ll /dev//group (look for 0xNN0000, were NN is the unique number to remember)
4. vgexport
5. mkdir /dev/
6. mknod /dev//group c 64 0xNN0000
7. vgimport -s -m .map /dev/

8. Test package failover.

[Chris Bishop]

Thanks for the reply, Stephen. All of my Serviceguard VGs have the following status:

available, exclusive

Ideally, I would like to add a new disk to each VG with vgextend that matches exactly one of the old disks, pvmove the data to the new disk, then vgreduce the old disk from the VG. Will that work without taking down the cluster or the packages? I realize I will need to remap my VGs and recreate them on the other node afterwards.

[Viktor Balogh]

Hi Chris,

>Ideally, I would like to add a new disk to each VG with vgextend that matches exactly one of the old disks, pvmove the data to the new disk, then vgreduce the old disk from the VG.

I would rather build an extra mirror to the LV, and after the synchronisation remove the old mirror half and reduce the VG by the defective/old LUNs. This way you would have a complete working copy of the FS all the time. I do not even want to think of a possible data loss in case pvmove would fail for some reason. Please consider it.

[Eric SAUBIGNAC]

Bonjour Chris,

Whatever method you use, be aware of CLUSTER_LOCK_VG and CLUSTER_LOCK_PV or CLUSTER_LOCK_LUN. If you use one of this method as split brain arbitration, you will have to stop the whole cluster to have locations moved to a new frame.

HTH

Eric

[Stephen Doud]

I agree with Viktor - it is safer to LVM mirror the new drive in and then unmirror the old drive and pvremove it. This will keep the data available for the running package, so you can keep the business critical application online.

Eric has a good point as well. Use 'cmviewconf | grep lock' to see if your cluster uses a lock VG/pv and if so, which it is. If one of the defective drives is the lock disk, you will have to update the cluster config file with a remaining pv, then halt the cluster eventually (unless this is A.11.19) to update the cluster binary with another drive in one of the cluster VGs.

[Chris Bishop]

I understand the mirroring technique is safer, but I have had problems in the past where the mirror to the new disk was added, but the removal of the mirror from the old disk did not work as expected. I have attached the details for the work I want to get done. If someone could look this over and provide the alternative commands for mirroring instead of pvmoving, I will certainly consider going that route. I am trying to get this done over the weekend.

Another one of my problems is the combination of the MAX PV and MAX PE settings on one of my large VGs will not allow me to add enough disks to mirror the LV. This is LV that contains the cluster lock disk so it must be mirrored. I will have to do some pvmoves in this VG on a smaller LV before I can add enough disks to mirror the larger LV.

As for the cluster lock disk, it is included in one of my VGs and I was not going to touch it. This disk would be part of the mirroring process in my attached file. The next scheduled downtime for this cluster is next weekend and I will reconfigure the cluster and change the cluster lock disk at that time.

[Viktor Balogh]

>but the removal of the mirror from the old disk did not work as expected.

What went wrong with lvreduce? Take a look at the allocation policy, and if any PE is used after lvreduce. If there's any PE allocated, the pv can't be removed from the vg.

[Chris Bishop]

Attached is the same workplan using mirrors to walk over to the new disks instead of pvmoves.

What do you guys think?

[Eric SAUBIGNAC · Accepted solution]

Chris,

Some remarks before leaving the office :

- You have forgotten to vgreduce the old alternate path /dev/dsk/c10tXdY. You have to do it

- You don't want to reduce mirror off of the cluster disk LV. Why not ? It doesn't matter wether or not you have data on the disk. The most important thing is that the disk exists in the VG at cm[apply|check]conf or if an arbitration is needed. So you can safely reduce the mirror.

What could be annoying, would be to vgreduce the pv_lock disk. But, again, it could MATTER ONLY if a split brain condition araise (or at cmcheckconf/cmapplyconf). I mean, if the cluster lock disk no more exists it has no impact on the running cluster. More, you can start the cluster even if the lock disk is missing.

Don't forget to modify cluster configuration with pv_lock definition, for both nodes, and cmapplyconf and I guess everything should go well

Eric