To support the EVM snapshot method of replication, the HSG controller and ACS require the following configuration:
ACS 8.5P or 8.5S
Fibre Channel switched fabric topology
Dual HSG controllers in transparent or multibus failover mode
512 MB of cache per controller
Mirrored cache
Additionally, the ACS imposes rules on the creation of snapshot units:
You cannot create more than four snapshot units per controller pair (subsystem)
You cannot create a snapshot of a snapshot
You cannot create multiple snapshots of the same source unit
A snapshot unit and its source unit must be online to the same controller
A snapshot unit and its source unit must be in the same unit number range of 0 to 199 or 100 to 199 (transparent failover only)
The logic to work within these rules is integrated into EVM. When EVM indicates that a snapshot is not possible, or if an EVM job that involves a snapshot fails, it is likely that one or more of these rules has not been satisfied.