HP Application Recovery Manager software A.06.10 Concepts guide (March 2008)

Standard snapshot
Figure 5 Creating a standard snapshot
1. At time T
0
, storage capacity equal to that taken up by the source volumes
concerned is allocated on the array for the target volumes.
No data is copied from the source storage blocks. Instead, pointers are mapped
to the storage blocks holding the original data and the copy is completely virtual.
From a host’s perspective, however, a complete replica of the source volumes
at time T
0
exists in the target volumes and it is ready for use.
2. After snapshot creation, if T
0
source data needs to be updated, it is first copied
to target storage blocks and pointers in the snapshot are remapped to these
copies. Only then is the source data updated.
This is known as “copy-on-write”.
3. The snapshot is now partly real (where source data has been copied) and partly
virtual. When the replica is accessed, any previously copied data is read from
the target storage blocks and any data that has not been copied is read from
the source storage blocks. From a host’s perspective, therefore, a complete
replica of the source data at time T
0
still exists.
Characteristics of standard snapshots
Replication techniques26