The SDM is designed to have minimal performance impact on the primary system by using a combination of buffering in the software and cache in the storage control. However, environmental or workload conditions can cause either or both of these resources to be consumed, which can result in application performance degradation.
A TIMEOUT value that is specified with the XSET command or with parmlib processing allows you control over application protection versus disaster recovery readiness. The TIMEOUT parameter is often referred to as the dead system data mover timer and is used as protection against an extended period of long busy. (Long busy is a disk subsystem response to a write I/O request.)
The SDM typically communicates with a disk subsystem 20–30 times per second in a busy environment, and at least once per second in an idle environment. Being unable to communicate for an extended period of time is an unusual situation that can be caused by a line outage, an extended busy lockout condition, or a problem with the SDM.
Each storage control session maintains an independent timer. In an environment where little or no cache updates occur, few cache resources are used by the SDM. In this case, the storage control session may remain active for significantly longer than the TIMEOUT value specified. The maximum time an application can be affected is determined by the TIMEOUT value.
For additional information about the TIMEOUT parameter, refer to Using the XSET TIMEOUT parameter.