Choosing Latency and SRL Protection

The replication parameters latencyprot and srlprot provide a compromise between synchronous and asynchronous characteristics. These parameters allow the Secondary to fall behind, but limit the extent to which it does so.

When latencyprot is enabled, the Secondary is only allowed to fall behind by a predefined number of requests, a latency high mark. After this user-defined latency high mark is reached, throttling is triggered. This forces all incoming requests to be delayed until the Secondary catches up to within another predefined number of requests, the latency low mark. Thus, the average write latency seen by the application increases. A large difference between the latency high mark and latency low mark causes occasional long delays in write requests, which may appear to be application hangs, as the SRL drains down to the latency low mark. Most other write requests remain unaffected. A smaller range spreads the delays more evenly over writes, resulting in smaller but more frequent delays. For most cases, a smaller difference is probably preferable.

The replication parameter srlprot can be used to prevent the SRL from overflowing and has an effect similar to latencyprot. However, the srlprot attribute is set to autodcm by default, which allows the SRL to overflow and convert to dcm_logging mode. As a result, there is no effect on write performance, but the Secondary is allowed to fall behind and is inconsistent while it resynchronizes. For more information, refer to the
VERITAS Volume Replicator Administrator's Guide.