Using zFS read/write sysplex-aware file systems

When you run zFS V2R1 in a shared file system environment, the zFS PFS runs as sysplex-aware. However, by default, each zFS file system is mounted as non-sysplex aware. zFS does allow zFS read/write file systems to run as sysplex-aware. But, this must be explicitly requested on a file system basis, using either the RWSHARE MOUNT PARM or the sysplex_filesys_sharemode=rwshare configuration option.

Consider which zFS read/write file systems you might want to be sysplex-aware. Good candidates are zFS read/write file systems that are accessed from multiple systems or are mounted with AUTOMOVE and might be moved by z/OS® UNIX (as a result of a shutdown or IPL) to systems that do not necessarily do the most accesses. Be aware that RWSHARE file systems use more virtual storage in the zFS address space and you must be careful that you do not exhaust the zFS address space storage. See the sample zFS query report STOR for information about monitoring storage usage in the zFS address space. Generally, the system-specific file system (and /dev, /etc, /tmp, /var) should be mounted as NORWSHARE and UNMOUNT because they typically are accessed only from the owning system.

zFS read-only mounted file systems are not affected by this support. However, if you remount a read-only file system to read/write (using the chmount command or the TSO/E UNMOUNT REMOUNT command), this is treated like a primary mount on the current z/OS UNIX owning system. In this case, MOUNT parameters (such as RWSHARE or NORWSHARE) or MOUNT defaults (such as the current sysplex_filesys_sharemode setting on that system) take affect when it is mounted read/write. When you remount back to read-only, those mount options are irrelevant again.

Note: These MOUNT parameters and MOUNT defaults do not take effect when a remount to the same mode is run.

The sysplex_filesys_sharemode option on a system specifies if a zFS read/write file system will be mounted as sysplex-aware when a MOUNT is issued on that system without specifying either NORWSHARE or RWSHARE in the MOUNT PARM. The default value for sysplex_filesys_sharemode is norwshare. This means that a MOUNT for a zFS read/write file system that does not have NORWSHARE or RWSHARE specified in the MOUNT PARM will result in the file system being non-sysplex aware. If you want zFS read/write mounts to be sysplex-aware, then specify sysplex_filesys_sharemode=rwshare. This option can be specified in the IOEFSPRM configuration options file and takes effect on the next IPL (or restart of zFS). It can also be specified dynamically with the zfsadm config -sysplex_filesys_sharemode command. Typically, you should specify the same sysplex_filesys_sharemode value on all your systems. Otherwise, z/OS UNIX file system ownership movement might change the sysplex-awareness of a file system that does not have NORWSHARE or RWSHARE specified in the MOUNT PARM.

Note: If any zFS read/write file systems were previously mounted as NORWSHARE, they will usually remain non-sysplex aware until they are un-mounted and then mounted back on the RWSHARE system. However, there are situations when the sysplex awareness may change. See Changing zFS attributes on a mounted zFS compatibility mode file system for more information.

Your sysplex root file system should be read-only. However, if your sysplex root file system is normally read/write, you should make it sysplex-aware. You cannot unmount the sysplex root file system so you need an alternative method. One method is to remount your sysplex root to read-only, move z/OS UNIX ownership of the file system, if necessary, to a system that has sysplex_filesys_sharemode=rwshare, and then remount the sysplex root back to read/write. You may want to update your ROOT statement in BPXPRMxx to add PARM('RWSHARE') to ensure that you do not lose the sysplex-aware attribute if the ROOT is mounted again. Note that, in this case, you may see a USS_PARMLIB health check message indicating that your BPXPRMxx ROOT PARM does not match your current sysplex root PARM. This is expected and is normal.