About The Snapshot Feature - HP P2000 G3 Reference Manual

About the Snapshot feature

Snapshot is a licensed feature that provides data protection by enabling you to create and save snapshots
of a volume. Each snapshot preserves the source volume's data state at the point in time when the snapshot
was created. Snapshots can be created manually or by using the task scheduler.
When the first snapshot is taken of a standard volume, the system automatically converts the volume into a
master volume and reserves additional space for snapshot data. This reserved space, called a snap pool,
stores pointers to the source volume's data. Each master volume has its own snap pool. The system treats a
snapshot like any other volume; the snapshot can be mapped to hosts with read-only access, read-write
access, or no access, depending on the snapshot's purpose. Any additional unique data written to a
snapshot is also stored in the snap pool.
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
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 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).
Snapshot-1
(Monday)
Snapshot-2
(Tuesday)
HP StorageWorks P2000 G3 MSA System SMU Reference Guide
Snap Pool-1
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