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InfoScale™ 9.0 Replication Administrator's Guide - AIX

Last Published: 2025-04-14

Product(s): InfoScale & Storage Foundation (9.0)

Platform: AIX

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. 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 latencyprot parameter can be effectively used to achieve the required Recovery Point Objective (RPO). Before setting the latencyprot parameter, consider the factors that affect the latency high mark and latency low mark values:

RPO in writes
Average write rate
Average available network bandwidth
Average write size
Maximum time required by the SRL to drain from the latency high mark to the latency low mark. This is the timeout value of the application which is the most sensitive, i.e., the application with the LOWEST timeout value among all using volumes from the RVG.
Number of writes already logged in the SRL

Based on specific requirements, set the user-defined latency high mark to an acceptable RPO value, in terms of number of writes. Thus, the value that should be set for the latency high mark is calculated as RPO in writes divided by average write size.

Set the latency low mark value such that the stalling of writes does not affect any application. Thus, assuming that the average network rate is greater than or equal to the average write rate calculate the effective SRL drain rate as average network rate - average write rate. Once this value is obtained the latency low mark value is calculated as:

latency high mark -(Effective SRL drain rate * lowest timeout)/ 
average write size

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.

See Protecting against SRL overflow.