MAXBUF parameter

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Maximum number of buffers


Default value
Parameter type
Where set
On User 0's parameter line
Related products
Model 204 V2.2 or earlier


MAXBUF specifies the maximum number of in-memory file page buffers that can be allocated below the bar (BTB) during Model 204 initialization.

The actual number of BTB pages allocated to the disk buffer pool can be identified by viewing the parameter NUMBUF.

Note: As of version 7.7 of Model 204, whether BTB buffers are used at all depends entirely on the setting of NUMBUFG, which activates buffer storage above the bar.

  • If NUMBUFG is greater than 0, all buffer pool buffers are forced above the bar — no buffers are below the bar. The MINBUF and NUMBUF parameters are forced to 0, and the other parameters that affect BTB storage (MAXBUF, SPCORE, LDKBMWND, and NLRUQ) are ignored.
  • If NUMBUFG is set to 0, all buffers are below the bar, and the settings of the BTB-related parameters are respected and calculated as in pre-7.7 versions.

If BTB buffers are used

When BTB buffers are being used (that is, NUMBUFG is 0 under version 7.7 or higher, or NUMBUFG is any value under Model 204 prior to 7.7):

  • If NLRUQ is set greater than 1, then the value of MAXBUF is rounded up to a multiple of NLRUQ. The maximum setting is 300000.
  • You must set the XMEMOPT X'02' bit to get more than about 15000-18000 actually allocated to NUMBUF.

    If the M204XSVC module is linked into the nucleus, XMEMOPT defaults to X'03' (as of version 7.4) or X'02' (as of version 7.7), so the MAXBUF setting should be achieved, provided that sufficient REGION is available.

  • If MAXBUF is set to a value less than MINBUF, MAXBUF is automatically reset to the value of MINBUF, and the following message is issued:


  • You can use the MONITOR DISKBUFF command to display, from each table of each file, the number of BTB pages that are currently located in the disk buffers.
  • If the prefetch feature is enabled (SEQOPT=1), then the MAXBUF parameter must be resized based on the following formula:

    MAXBUF = NUSERS * (4 + 2 * (Maximum FOR EACH RECORD loop nest level))

    The prefetch feature is described in The prefetch (look-ahead read) feature and in Prefetch feature.