Full-sized instant snapshots are a variation on the third-mirror volume snapshot model that make a snapshot volume available for I/O access as soon as the snapshot plexes have been created.

Figure: Full-sized instant snapshot creation and usage in a backup cycle shows the full-sized instant volume snapshot model.

To create an instant snapshot, use the vxsnap make command. This command can either be applied to a suitably prepared empty volume that is to be used as the snapshot volume, or it can be used to break off one or more synchronized plexes from the original volume (which is similar to the way that the vxassist command creates its snapshots).

Unlike a third-mirror break-off snapshot created using the vxassist command, you can make a backup of a full-sized instant snapshot, instantly refresh its contents from the original volume, or attach its plexes to the original volume, without completely synchronizing the snapshot plexes from the original volume.

VxVM uses a copy-on-write mechanism to ensure that the snapshot volume preserves the contents of the original volume at the time that the snapshot is taken. Any time that the original contents of the volume are about to be overwritten, the original data in the volume is moved to the snapshot volume before the write proceeds. As time goes by, and the contents of the volume are updated, its original contents are gradually relocated to the snapshot volume.

If a read request comes to the snapshot volume, yet the data resides on the original volume (because it has not yet been changed), VxVM automatically and transparently reads the data from the original volume.

If desired, you can perform either a background (non-blocking) or foreground (blocking) synchronization of the snapshot volume. This is useful if you intend to move the snapshot volume into a separate disk group for off-host processing, or you want to turn the snapshot volume into an independent volume.

The vxsnap refresh command allows you to update the data in a snapshot, for example, before taking a backup.

The command vxsnap reattach attaches snapshot plexes to the original volume, and resynchronizes the data in these plexes from the original volume. Alternatively, you can use the vxsnap restore command to restore the contents of the original volume from a snapshot that you took at an earlier point in time. You can also choose whether or not to keep the snapshot volume after restoration of the original volume is complete.

By default, the FastResync feature of VxVM is used to minimize the time and I/O needed to resynchronize the data in the snapshot mirror. FastResync must be enabled to create instant snapshots.

An empty volume must be prepared for use by full-sized instant snapshots and linked break-off snapshots.