Environment
PlateSpin Forge 3.x and above
PlateSpin Migrate 9.x and above
Situation
The use of LVM snapshots is the default way of migrating hard drive partitions on LVM logical volumes. Snapshots can be taken quickly and allow for greatest inter-volume consistency.
For an LVM snapshot to be created, there needs to be available free space in the LVM volume group of the logical volume to be transferred. Free space in an LVM volume group is space that has not been allocated to any logical volume. The free space within each logical volume is not available for LVM snapshot usage.
The amount of space allocated to each LVM snapshot is based on its relative characteristics (eg. size, usage). The software will attempt to create snapshots of the appropriate size that will maximize the chance of success during the transfer. In some cases, this may not be sufficient.
1. In some cases, a snapshot of a logical volume will not be created due to insufficient free space. In this case, the data transfer will automatically fallback to using the block-based driver monitoring method for that particular logical volume. (NotEnoughSpaceToCreateLvmSnapshot)
2. In some cases, the rate of change of data on the volume may be greater than the LVM snapshot can hold during the volume transfer. If this happens, the snapshot will be invalidated and the transfer will fail. (LvmSnapshotFailure)
Resolution
To resolve either of these possible issues, the system administrator can do one or both of the following:
1. Increase the size of the free space on the volume group. This would typically involve adding a new hard drive to the source workload and extending the volume group to include the new hard drive. Once this is done, the system will be able to make a better decision on snapshot sizes.
2. Reserve space for use by a snapshot of a logical volume. This space will be used as the backing store for the snapshot of the logical volume during migration. To reserve free space, the system administrator must create a new logical volume with a name suffix of "-PS-user-snapshot" in the same volume group. At the beginning of the data transfer, the volume will be temporarily replaced by a snapshot of the same size. At the completion of the data transfer, the snapshot will be deleted, and the “reserved” volume will be recreated.
For example, to reserve 200MB for an LVM snapshot for a volume named “/dev/myVG/myLV”, the system administrator must create a new volume with the name “/dev/myVG/myLV-PS-user-snapshot”.
# lvcreate -L 200MB -n myLV-PS-user-snapshot myVG
This will create a 200MB volume named myLV-PS-user-snapshot on the myVG volume group.