HP MSA 1040 HP MSA 1040 SMU Reference Guide (762784-001, March 2014) - Page 23

the snapshot data reverts to the original data that was snapped. This feature is useful for testing an application

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The following figure shows how the data state of a master volume is preserved in the snap pool by two snapshots taken at different points in time. The dotted line used for the snapshot borders indicates that snapshots are logical volumes, not physical volumes as are master volumes and snap pools. MasterVolume-1 Snapshot-1 (Monday) Snap Pool-1 Snapshot-2 (Tuesday) Figure 1 Relationship between a master volume and its snapshots and snap pool The snapshot feature uses the single copy-on-write method to capture only data that has changed. That is, if a block is to be overwritten on the master volume, and a snapshot depends on the existing data in the block being overwritten, the data is copied from the master volume to the snap pool before the data is changed. All snapshots that depend on the older data are able to access it from the same location in the snap pool; this reduces the impact of snapshots when writing to a master volume. In addition, only a single copy-on-write operation is performed on the master volume. The storage system allows a maximum number of snapshots to be retained, as determined by an installed license. For example, if your license allows four snapshots, when the fifth snapshot is taken an error message informs you that you have reached the maximum number of snapshots allowed on your system. Before you can create a new snapshot you must either delete an existing snapshot, or purchase and install a license that increases the maximum number of snapshots. The snapshot service has two features for reverting data back to original data: • Deleting only modified data on a snapshot. For snapshots that have been made accessible as read-write, you can delete just the modified (write) data that was written directly to a snapshot. When the modified data is deleted, the snapshot data reverts to the original data that was snapped. This feature is useful for testing an application, for example. You might want to test some code, which writes data to the snapshot. Rather than having to take another snapshot, you can just delete any write data and start again. • Rolling back the data in a source volume. The rollback feature enables you to revert the data in a source volume to the data that existed when a specified snapshot was created (preserved data). Alternatively, the rollback can include data that has been modified (write data) on the snapshot since the snapshot was taken. For example, you might want to take a snapshot, mount/present/map that snapshot for read/write, and then install new software on that snapshot for test purposes. If the software installation is successful, you can rollback the master volume to the contents of the modified snapshot (preserved data plus the write data). System concepts 23

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System concepts
23
The following figure shows how the data state of a master volume is preserved in the snap pool by two snapshots
taken at different points in time. The dotted line used for the snapshot borders indicates that snapshots are logical
volumes, not physical volumes as are master volumes and snap pools.
Figure 1
Relationship between a master volume and its snapshots and snap pool
The snapshot feature uses the single copy-on-write method to capture only data that has changed. That is, if a block
is to be overwritten on the master volume, and a snapshot depends on the existing data in the block being
overwritten, the data is copied from the master volume to the snap pool before the data is changed. All snapshots
that depend on the older data are able to access it from the same location in the snap pool; this reduces the impact
of snapshots when writing to a master volume. In addition, only a single copy-on-write operation is performed on the
master volume.
The storage system allows a maximum number of snapshots to be retained, as determined by an installed license. For
example, if your license allows four snapshots, when the fifth snapshot is taken an error message informs you that you
have reached the maximum number of snapshots allowed on your system. Before you can create a new snapshot you
must either delete an existing snapshot, or purchase and install a license that increases the maximum number of
snapshots.
The snapshot service has two features for reverting data back to original data:
•
Deleting only modified data on a snapshot. For snapshots that have been made accessible as read-write, you can
delete just the modified (write) data that was written directly to a snapshot. When the modified data is deleted,
the snapshot data reverts to the original data that was snapped. This feature is useful for testing an application,
for example. You might want to test some code, which writes data to the snapshot. Rather than having to take
another snapshot, you can just delete any write data and start again.
•
Rolling back the data in a source volume. The rollback feature enables you to revert the data in a source volume
to the data that existed when a specified snapshot was created (preserved data). Alternatively, the rollback can
include data that has been modified (write data) on the snapshot since the snapshot was taken. For example, you
might want to take a snapshot, mount/present/map that snapshot for read/write, and then install new software
on that snapshot for test purposes. If the software installation is successful, you can rollback the master volume to
the contents of the modified snapshot (preserved data plus the write data).
MasterVolume-1
Snap Pool-1
Snapshot-1
Snapshot-2
(Monday)
(Tuesday)