Full-Sized Instant Snapshots

Full-sized instant snapshots are a variation on the third-mirror volume snapshot model that make a snapshot volume available for access as soon as the snapshot plexes have been created. The full-sized instant volume snapshot model is illustrated in Full-Sized Instant Snapshot Creation and Usage in a Backup Cycle.

Full-Sized Instant Snapshot Creation and Usage in a Backup Cycle

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To create an instant snapshot, you 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 needing to completely synchronize 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 preserved on 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 desired, you can additionally select to 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 use the vxsnap dis or vxsnap split commands to turn the snapshot volume into an independent volume.

The vxsnap refresh command allows you to update the data in a snapshot each time that you make a backup.

The command, vxsnap reattach, can be used to attach snapshot plexes to the original volume, and to resynchronize 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. See Restoring the Original Volume from a Snapshot for more information.

By default, the FastResync feature of VxVM is used to minimize the time needed to resynchronize the data in the snapshot mirror. If FastResync is not enabled, a full resynchronization of the data is required. For details, see FastResync.

See Creating Instant Snapshots for details of the procedures for creating and using this type of snapshot.

For information about how to prepare an empty volume for use by full-sized instant snapshots, see Creating a Volume for Use as a Full-Sized Instant Snapshot.