Tuning z/OS UNIX performance in a sysplex

The intersystem communication that is required to provide the additional availability and recoverability associated with z/OS UNIX shared file system support, affects response time and throughput on read/write file systems being shared in a sysplex.

For example, assume that a user on SY1 requests a read on a file system mounted read/write and owned by SY2. Using shared file system support, SY1 sends a message requesting this read to SY2 via an XCF messaging function:

SY1 ===> (XCF messaging function) ===> SY2

After SY2 gets this message, it issues the read on behalf of SY1, and gathers the data from the file. It then returns the data via the same route the request message took:

SY2 ===> (XCF messaging function) ===> SY1

Thus, adding z/OS UNIX to a sysplex increases XCF message traffic. To control this traffic, closely monitor the number and size of message buffers and the number of message paths within the sysplex. It is likely that you will need to define additional XCF paths and increase the number of XCF message buffers above the minimum default. For more information about tuning the signal services in a sysplex environment, see Tuning the signaling services and coupling facility in z/OS MVS Setting Up a Sysplex.

Be aware that because of I/O operations to the CDS, every mount request requires additional system overhead. Mount time increases as a function of the number of mounts, the number of members in a sysplex, and the size of the CDS. You will need to consider the effect on your recovery time if many mounts are required on any system participating in a shared file system.