ALTER BUFFPOOL on z/OS

Use the MQSC command ALTER BUFFPOOL to dynamically change the settings of a predefined buffer pool on z/OS®.


Use MQSC commands

For information on how we use MQSC commands, see Performing local administration tasks using MQSC commands.

Parameters not specified in the ALTER BUFFPOOL command result in the existing values for those parameters being left unchanged.

We can issue this command from sources 2CR. For an explanation of the source symbols, see Use commands on z/OS.


Syntax diagram

Synonym: ALT BP

ALTER BUFFPOOL

ALTER BUFFPOOL ( buf-pool-id ) BUFFERS(integer)LOC(BELOW)LOCATION(ABOVE)PAGECLAS(4KB)PAGECLAS(FIXED4KB)


Usage notes for ALTER BUFFPOOL

  1. Buffers are added or removed according to whether the value is more than or less than the current allocation (which can be shown by the DISPLAY USAGE command).
  2. If there is insufficient storage, of the type specified by the PAGECLAS attribute to add the requested number, as many as possible are added.
  3. The command runs asynchronously. Message CSQP023I is sent to the console when the command is complete.
  4. ALTER BUFFPOOL cannot be issued from CSQINPT.
  5. If we have made changes to buffer pools by using the ALTER BUFFPOOL command, particularly if we have reduced the buffer pools by large quantities, you should recycle the queue manager as soon as possible, to clear up any storage fragmentation caused by the change in buffer pool size.

    Failure to recycle the queue manager, might result in you receiving the following error code, ABEND878-10 - Virtual private region depleted, caused by the fragmentation of region storage in the IBM MQ MSTR address space.


Parameter descriptions for ALTER BUFFPOOL

    (buf-pool-id)
    Buffer pool identifier.

    If IBM MQ Version 8.0 new functions are enabled with OPMODE, this parameter is an integer in the range zero through 99. Otherwise, this parameter is an integer in the range zero through 15.

    BUFFERS(integer)
    This parameter is optional and is the number of 4096 byte buffers to be used in this buffer pool.

    If the value of the LOCATION parameter is BELOW, the minimum value of buffers is 100 and the maximum value is 500,000. If the value of the LOCATION parameter is ABOVE, then valid values are in the range of 100 to 999999999 (nine nines). The storage used for buffers in a buffer pool with LOCATION ABOVE is obtained in multiples of 4MB. Therefore specifying a BUFFERS value which is a multiple of 1024 will make the most efficient use of storage.

    See Buffers and buffer pools for guidance on the number of buffers we can define in each buffer pool.

    When defining a buffer pool care should be taken to ensure that there is sufficient storage available for it either above or below the bar. For more information, see Address space storage.

    Note: Creating a large buffer pool can take several minutes depending on size of the buffer pool and machine configuration. In some cases message CSQP061I might be output.

    LOCATION(LOC)(BELOW or ABOVE)
    LOCATION and LOC are synonyms and either, but not both, can be used.

    The LOCATION or LOC parameter specifies where the memory used by the specified buffer pool is located.

    This memory location can be either ABOVE (64 bit) or BELOW (31 bit) the bar. Valid values for this parameter are BELOW or ABOVE, with BELOW being the default.

    ABOVE can only be specified if IBM MQ Version 8.0 new functions are enabled with OPMODE. BELOW can be specified regardless of the value of OPMODE and has the same effect as not specifying the LOCATION parameter.

    When altering a buffer pool, you should take care to make sure that there is sufficient storage available if increasing the number of buffers, or changing the LOCATION value. Switching the location of the buffer pool can be a CPU and I/O intensive task. You should perform this task when the queue manager is not being heavily used.

    For more information, see Address space storage.

    PAGECLAS(4KB or FIXED4KB)

    Optional parameter that describes the type of virtual storage pages used for backing the buffers in the buffer pool.

    This attribute applies to all buffers in the buffer pool, including any that are added later as a result of using the ALTER BUFFPOOL command. The default value is 4KB, which means that pageable 4KB pages are used to back the buffers in the pool.

    4KB is the only valid value if the buffer pool has its location attribute set to BELOW. If the buffer pool has its LOCATION attribute set to ABOVE, it is also possible to specify FIXED4KB. This means that fixed 4KB pages, which are permanently in real storage and will never be paged out to auxiliary storage, are used to back the buffers in the buffer pool.

    FIXED4KB can only be specified if IBM MQ Version 8.0 new functions are enabled with OPMODE, whereas 4KB can be specified regardless of the value of OPMODE.

    The PAGECLAS attribute of a buffer pool can be altered at any time. However, the alteration only takes place when the buffer pool switches location from above the bar, to below the bar, or the other way round. Otherwise, the value is stored in the log of the queue manager and is applied when the queue manager next restarts.

    The current value of PAGECLAS can be checked by issuing the DISPLAY USAGE PSID(*) command. Doing this also results in a CSQP062I message being output, if the current value of PAGECLAS is different from the value in the log of the queue manager.

    For example:

    • Buffer pool 7 currently has LOCATION(ABOVE) and PAGECLAS(4KB) specified. If ALTER BUFFPOOL(7) PAGECLAS(FIXED4KB) is specified, the buffer pool continues to be backed by pageable 4KB pages as the LOCATION has not been changed.
    • Buffer pool 8 currently has LOCATION(BELOW) and PAGECLAS(4KB) specified. If ALTER BUFFPOOL(8) LOCATION(ABOVE) PAGECLAS(FIXED4KB) is specified, the buffer pool is moved above the bar and has its buffers backed by fixed 4KB pages, if any are available.

    When you specify PAGECLAS(FIXED4KB) the whole buffer pool is backed by page-fixed 4KB pages, so ensure that there is sufficient real storage available on the LPAR. Otherwise, the queue manager might not start, or other address spaces might be impacted; for more information, see Address space storage.

    See IBM MQ Support Pac MP16: IBM MQ for z/OS - Capacity planning & tuning for advice on when to use the FIXED4KB value of the PAGECLAS attribute.

Attention: The queue manager records the current buffer pool settings in checkpoint log records. These buffer pool settings are automatically restored when a queue manager is later restarted. This restoration occurs after processing of the CSQINP1 data set. Therefore, if we have used ALTER BUFFPOOL since the buffer pool was last defined, any DEFINE BUFFPOOL command in CSQINP1 has been ignored at restart, unless the REPLACE attribute has been specified.