3PAR InForm® OS 2.3.1 Concepts Guide (320-200112 Rev B, February 2010)

10.9
Virtual Copy Snapshots
InForm OS Version 2.3.1 3PAR InForm OS Concepts Guide
10.4.1.1 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 10-3 for an example of a sequence of snapshots.
Figure 10-3. Snapshot Tree
The relationships between the virtual copies derived from a base volume can be represented as
a tree. In the example in Figure 10-3, the base volume BaseVV is the starting point. In this
example, each new virtual copy of the original has its name incremented by 1.
BaseVV
(RW)
S0
(RO)
S1
(RO)
S2
(RO)
Parent
Parent
S1_0
(RW)
Parent
Copy of
S1_0_1
(RO)
Parent
Copy of
S1_0_0
(RO)
Copy of
Copy of
Parent
Copy of
Parent
Copy of
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.