When a file system is mounted read-only (such as on SY2), the mount request is sent to the local physical file system (in this case, zFS) and zFS opens the file system data set (for read). If the mount is successful on that system, z/OS® UNIX records the mount and sends a signal to the other sysplex member systems to issue a “catch-up” mount on each system. Each z/OS UNIX on each other system then reads the couple data set (CDS) and determines that it needs to send a mount request to the local zFS for that file system. Each “local mount” causes zFS to open the data set (for read). In this way, the mount on SY2 causes the file system to be mounted on every member of the sysplex.

Figure 1. Sysplex-aware file system (read-only)

For read-only mounted file systems, file requests are sent directly to the local physical file system, which directly reads the file system data on DASD (see Figure 1). That means each zFS on each system has the zFS file system opened (for read) and directly accesses the data. Read-only mounted file systems are referred to as being sysplex-aware.