Storage systems offer functions for remote replication between geographically separated storage systems.
The software product SHC-OSD is the BS2000 host component for storage systems. It provides information services and commands for controlling the replication functions of the storage systems. A detailed description of the replication functions is contained in the “SHC-OSD” manual [29 (Related publications)].
Performance behavior in the case of synchronous remote replication
If a copy of the data is kept in a remote second storage system, each write access is entered first in the local cache and then in the "remote cache".
This means that using synchronous remote replication at least doubles the hardware service time for write accesses. An increased protocol time for processing the function also has to be taken into account.
Another major factor is the distance between the locations. Double the runtime of light in glass (outward and return directions) must be added in each case.
The write hit rate is usually 100% (provided there are sufficient “breathers” for saving the cache contents), i.e. each write access is performed quickly. With small blocks, the doubling of the hardware service time is normally of no consequence, but with large blocks (e.g. copy operations) there is a clear increase in the hardware service time.
An additional increase in the hardware service time can occur if the performance capability of the remote data links is insufficient.