HP 3PAR StoreServ Concepts Guide: HP 3PAR OS 3.1.3

Copy-on-Write Function
When a virtual volume or snapshot’s source volume is written to, the copy-on-write function preserves
the data that is to be overwritten. The data is copied to the snapshot space associated with the
original virtual volume before the write operation is completed, and a pointer in the administration
space points to the copied data.
See Figure 11 (page 49) for an example of a sequence of snapshots.
Figure 11 Snapshot Tree
In Figure 11 (page 49):
• S0 is the first virtual copy made of BaseVV.
• S2 is the most recent virtual copy.
• Each copy tracks changes made to BaseVV from its own creation date until the next snapshot
is made.
• S1_0 can be created at any time after S1 is created.
The relationships between the virtual copies derived from a base volume can be represented as a
tree. In the example in Figure 11 (page 49), the base volume BaseVV is the starting point. In this
example, each new virtual copy of the original has its name incremented by 1.
Each copy of a copy has an additional level added to its name: in this example, the first copy of
S1 is S1_0, and a copy of S1_0 is S1_0_0. Unlike the automatic snapshots created for physical
copies, these snapshots are not assigned names by the system.
NOTE: The naming convention used in the example above is recommended, but it is not enforced
by the system. You can name each virtual volume and virtual copy at the time of creation.
The following rules are enforced by the system when you create a snapshot:
• The tree grows in alternating layers of read/write and read-only snapshots. You can only
make a read-only copy of a read/write volume, and you can only make a read/write copy
of a read-only volume.
• A maximum of 256 read/write virtual copies can be made from one read-only virtual volume.
Virtual Copy Snapshots 49