The XSET TIMEOUT keyword specifies the maximum application impact that is allowed by an XRC session. As a result of the TIMEOUT value, a storage control that supports hardware bitmapping automatically suspends an XRC session if the SDM does not read from the storage control sidefile within the timeout period. This action frees up storage control resources. The hardware bitmap and application programs continue with no performance impact. XRC resynchronizes the volumes, based on the hardware bitmap, when it receives a subsequent XADDPAIR command for the volumes on that storage control. The volumes are not consistent with the session consistency group time until the resynchronization is complete.
A storage control that does not support hardware bitmapping ends the storage control session if the timeout interval expires. This frees all storage control resources so that there is no application performance impact. The fact that the storage control session has ended, however, means that you must completely synchronize the volumes in the XRC session from scratch. The volumes are not consistent with the XRC session consistency group time until the resynchronization is complete.
If you set the XSET TIMEOUT value low, protection is provided for applications at the expense of disaster recovery protection because session suspension or termination requires volume resynchronization to restore disaster protection. After the suspension or termination is done, all data on the secondary volumes is consistent with the reported timestamp.
If you set the XSET TIMEOUT value high, an XRC environment may avoid being suspended, but application slowdown might be experienced if the SDM stops reading updates from the primary subsystems, until the specified time has expired.
Recommendation: For IBM® storage subsystems, it is recommended that you not set the XSET TIMEOUT value lower than the missing interrupt handler (MIH) value for the primary volumes and never set it lower than the absolute minimum, 20 seconds. The default value is 5 minutes. The TIMEOUT interval does not begin until the residual in the primary storage control exceeds a high threshold value. You can minimize impact to application programs with the TIMEOUT value, application update rate, and the storage control cache size. Most customers specify the TIMEOUT value on the XSET command in the 30–90 second range.