This section is designed to help you understand the global resource
serialization latches that the z/OS UNIX System Services logical file
system (LFS) uses to provide serialization for file systems. It also
contains procedure to help you diagnose and resolve mount latch
contention and file system latch contention in
this section. See Procedure: Diagnosing and resolving latch contention.
The
z/OS UNIX System Services LFS uses three
levels of global resource serialization latches to provide serialization
for file systems:
- Mount
latch: The mount latch provides serialization for operations involving
the LFS and is the latch number two in the SYS.BPX.A000.FSLIT.FILESYS.LSN
latch set. The mount latch is obtained exclusively:
- When a file system is mounted or unmounted.
- In a sysplex configuration, for operations such as file system
moves, lost system recovery, system initialization, and reading from
or writing to a couple data set.
Obtaining the mount latch exclusively ensures that only one of
these activities is going on at the same time. Use the DISPLAY
GRS,LATCH,CONTENTION command to look for mount latch contention.
- File system latch: There is a
latch for each file system mounted. These latches are within the SYS.BPX.A000.FSLIT.FILESYS.LSN
latch set. The file system latch is:
- Obtained exclusively every time that file system is unmounted,
synchronized, exported or unexported by the server message block (SMB)
server, moved or recovered within a sysplex.
- Obtained in shared mode for the duration of any operation within the
file system, such as reads from or writes to a file. This prevents
the file system from being unmounted or moved, for example, while
there is an operation in progress on a file within the file system.
Use the DISPLAY GRS,LATCH,CONTENTION command
to look for file system latch contention.
- File
latch: There is file latch associated with each active file or
directory. A file latch can be obtained in either exclusive or shared
mode, depending on the operation involved. For example, the file latch
for a directory would be obtained in shared mode to read a name from
the directory. But it would be obtained exclusively to write a name
to the directory during a file create operation.
File
latches are not used with the z/OS® File
System (zFS) physical file system because the zFS has its own file
level serialization mechanisms. File latches are used with shared file system, TFS, pipes, character special,
and NFS client physical file systems.
File latches are in a
special group of latches with names in the form of SYS.BPX.A000.FSLIT.LSN.nn,
where nn is a hexadecimal number.
Use the DISPLAY
GRS,LATCH,CONTENTION command to look for file latch contention.
In addition to these three levels, the LFS also uses a
quiesce
latch, which is assigned to any file system that is:
- Quiesced by the BPX1QSE callable service, which is used by HSM
and other utilities to backup or dump file systems.
- For sysplex operations that operate against the file system as
a whole, such as moving and recovering.
When a file system is quiesced, normal operations are suspended,
and threads wait suspended for the file system's quiesce latch. The
system may hold the quiesce latch for longer than the duration of
a system call. Note that HSM does not use the quiesce latch for zFS
file systems.
Use the DISPLAY OMVS,FILE command to look for
quiesce latch contention on your system.
Diagnosing latch contention: You will know that you have
a case of latch contention by symptoms such as the following:
- One
or more systems issue message BPXM056E z/OS UNIX
SYSTEM SERVICES LATCH CONTENTION DETECTED
- z/OS UNIX System Services users are hung
- z/OS UNIX System Services itself seems
to be hung
In general, the key to resolving latch contention lies in finding
the latch holder. There are two kinds of
z/OS UNIX System Services tasks that
can hold a latch, potentially causing contention:
- User programs: When a user program invokes a file operation,
the system obtains the file system latch and possibly the file latch,
and holds the latches for the duration of the operation.
- z/OS UNIX System services (OMVS task): z/OS UNIX System Services may hold the
mount latch or a file system latch for more extended periods of time
for operations such as system recovery and file system moves.