Each plex or mirror of a volume is a complete copy of the data. When a plex is attached to a volume, the data in the plex must be synchronized with the data in the other plexes in the volume. The plex that is attached may be a new mirror or a formerly attached plex. A new mirror must be fully synchronized. A formerly attached plex only requires the changes that were applied since the plex was detached.
The following operations trigger a plex synchronization:
Moving or copying a subdisk with the vxsd command. The operation creates a temporary plex that is synchronized with the original subdisk.
Adding a mirror with the vxassist mirror command.
Creating a volume with a mirror with the vxassist make command.
Manually reattaching a plex with the vxplex att command.
Recovering a volume with the vxrecover command.
Adding a mirror to a snapshot with the vxsnap addmir command.
Reattaching or restoring a snapshot with the vxsnap command.
Plex synchronization can be a long-running operation, depending on the size of the volume and the amount of data that needs to be synchronized. Veritas Volume Manager provides several features to improve the efficiency of synchronizing the plexes.
If the FastResync feature is enabled, VxVM maintains a FastResync map on the volume. VxVM uses the FastResync map to apply only the updates that the mirror has missed. This behavior provides an efficient way to resynchronize the plexes.
The SmartMove™ feature reduces the time and I/O required to attach or reattach a plex to a VxVM volume with a mounted VxFS file system. The SmartMove feature uses the VxFS information to detect free extents and avoid copying them.
When the SmartMove feature is on, less I/O is sent through the host, through the storage network and to the disks or LUNs. The SmartMove feature can be used for faster plex creation and faster array migrations.
Recovery for synchronization tasks
In this release, VxVM tracks the plex synchronization for the following commands: vxplex att, vxassist mirror, vxsnap addmir, vxsnap reattach, and vxsnap restore. If the system crashes or the vxconfigd daemon fails, VxVM provides automatic recovery for the synchronization task. When the system is recovered, VxVM restarts the synchronization from the point where it failed. The synchronization occurs in the background, so the volume is available without delay.