Tracing the z/OS ZFS file system, and using ZFS commands.

I was looking into a little Java problem, and wanted to know which files were being used by my Java program. The “obvious” answer was a trace – but the IBM documentation was about 8 years out of date!

The key lesson from this post is to use commands like

f OMVS,PFS=ZFS,… instead of MODIFY ZFS,… if you have ZFS in your OMVS address space.

zFS running in the z/OS UNIX address space says

In releases before z/OS V2R2, the amount of 31-bit virtual storage that was needed by both z/OS UNIX and zFS combined would have exceeded the size of a 2 GB address space. Due to that size limitation, zFS and z/OS UNIX could not coexist in the same address space.

In z/OS V2R2, zFS caches are moved above the 2 GB bar into 64-bit storage. You can now choose to have zFS run in its own colony address space or in the address space that is used by z/OS UNIX, which is OMVS.

When running zFS in the OMVS address space, each file system vnode operation (such as creating a directory entry, removing a directory entry, or reading from a file) will have better overall performance. Each operation will take the same amount of time while inside zFS itself. The performance benefit occurs because z/OS UNIX can call zFS for each operation in a more efficient manner.

Some inherent differences exist when zFS is run in the OMVS address space.

MODIFY commands must be passed to zFS through z/OS UNIX. Use the form MODIFY OMVS,pfs=zfs,cmd. For more information, see the section on passing a MODIFY command string to a physical file system (PFS) through a logical file system (LFS) in z/OS MVS System Commands. This form of the MODIFY command can be used whether zFS is in its own address space or in the OMVS address space.

Issuing commands

So when the documentation says issue a command

Steps for tracing on zFS

If you are re-creating a problem and need to collect a zFS trace, use the following steps:

1. Allocate the trace output data set as a PDSE, RECFM=VB, LRECL=133 with a primary allocation of at least 50 cylinders and a secondary allocation of 30 cylinders.

2. Define the zFS trace output data set to zFS by either using the IOEFSPRM trace_dsn option, or dynamically by using the zfsadm config -trace_dsn command. If you use the IOEFSPRM option, zFS must be stopped and then restarted to pick up the change, unless you also dynamically activate the trace output data set with the zfsadm config -trace_dsn command.

3. When you are ready to re-create the problem, reset the zFS trace table using the MODIFY ZFS,TRACE,RESET command.

4. Re-create the problem.

5. Enter the MODIFY ZFS,TRACE,PRINT command. This formats and prints the trace table to the PDSE defined on the trace_dsn option.

You still use the Unix command to define the output destination of the trace

zfsadm config -trace_dsn ‘IBMUSER.ZFSTRACE’

but you use the following console command to cause the trace to be formatted to the file from the internal buffer.


Easy when you know how…

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