About Volume Snapshots

A snapshot is a copy of the data on a volume at the moment you create the snapshot. Creating a snapshot causes the array controller tray to suspend input/output (I/O) to the base volume while it creates a physical volume, called the reserve volume. The reserve volume stores information about the data that has changed since the snapshot was created. The capacity of the reserve volume is a configurable percentage of the base volume.

Snapshot is a premium feature that requires a license. You must enable the snapshot license before you can use the snapshot feature. For information on licenses, see Managing Licenses.

You work with a snapshot as you would with any other volume, with the exception that you cannot take a snapshot of a snapshot. Each snapshot can be accessed independently by other applications. A snapshot can be mounted on another server and used in the following ways:

Snapshots on the Sun StorEdge 6130 array are copy-on-write or dependent copies. In this type of snapshot, write operations to the primary volume causes the management software to copy the snapshot metadata and copy-on-write data to the reserve volume. Because the only blocks that are physically stored in the reserve volume are those that have changed since the time the snapshot was created, the snapshot uses less disk space than a full physical copy.

When a write operation occurs on the primary volume to a data block in which the data has not changed since the snapshot was created, the management software does the following:

When a data host sends a read request to the snapshot, the management software checks whether the requested blocks have changed on the primary volume since the snapshot was created. If they have changed, the read request is satisfied from the data stored in the snapshot reserve volume. If blocks have not changed, the read request is satisfied from the primary volume. Snapshots can also accept write operations. Write operations to a snapshot are stored in the snapshot reserve volume.

The management software provides a warning message when the reserve volume nears the threshold, which is a configurable percentage of the full capacity of the snapshot reserve (the default is 50%). When the reserve volume threshold is met, the reserve volume's capacity can be expanded using the free capacity on the virtual disk.

As long as a snapshot is enabled, storage array performance is affected by the copy-on-write activity to the associated reserve volume. If a snapshot is no longer needed, you can stop the copy-on-write activity by either disabling or deleting the snapshot.

When a snapshot is disabled, it and its associated reserve volume still exist. When you need to create a different point-in-time image of the same primary volume, you can resnap the volume to reuse the disabled snapshot and its associated reserve volume. This takes less time than creating a new snapshot.

If you do not intend to re-create a snapshot, you can delete the snapshot instead of disabling it. When you delete a snapshot, the management software also deletes the associated reserve volume.

To see the current snapshots for a particular volume, go to the Additional Information section of Snapshot Summary page for that volume, as described in Displaying Volume Snapshot Information.

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