A ShadowImage configuration consists of the following components:
- VSP One Block storage system
- ShadowImage software on the storage system
- Hosts connected to the storage system
- ShadowImage pairs consisting of primary volumes (P-VOLs) and secondary volumes (S-VOLs)
- Cascaded pairs
- Consistency groups (CTGs)
The following figure shows a typical ShadowImage configuration.
Pairs
A volume pair consists of a P-VOL and up to three S-VOLs. Because S-VOLs are updated asynchronously, the P-VOL and its associated S-VOLs might not be identical except immediately after the pair is split. After a pair is split, any further updates to the P-VOL are not copied to the S-VOL. Splitting or deleting a pair allows the host to access the S-VOL.
Cascaded pairs
ShadowImage supports two layers of pairs, layer-1 (L1) pairs and layer-2 (L2) pairs. Cascaded pairs are volume pairs created in the first and second layers. A pair made up of an L1 S-VOL and an L2 S-VOL is an L2 pair. You can pair each SI L1 P-VOL with up to three L1 S-VOLs. You can pair each SI L1 S-VOL with up to two L2 S-VOLs. This means that you can pair nine L1 and L2 S-VOLs with a single L1 P-VOL.
The following figure shows the structure of cascaded pairs.
In cascaded pair configurations, the P-VOL of an L1 pair is called a root volume, and the L1 S-VOL is called a node volume. The P-VOL for an L2 pair is the S-VOL of an L1 pair, a node volume, and the L2 S-VOL is called a leaf volume.
Consistency groups (CTGs)
If you want to manage multiple ShadowImage pairs as a single group, you can add the pairs to a consistency group (CTG). Consistency groups allow you to perform tasks on all of the pairs in the group at the same time, including splitting pairs and resynchronizing pairs. Performing tasks on a CTG ensures the consistency of the pair status for all pairs in the group.
(VSP One Block 85) When you use a CTG with CCI, you do not need to reserve the CTG.