MAXBUF parameter

Maximum number of buffers

Summary

Default value

256

Parameter type

System

Where set

On User 0's parameter line

Related products

All

Introduced

Model 204 V2.2 or earlier

Description

The maximum number of in-memory file page buffers that can be allocated during Model 204 initialization

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

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. Also, you must set XMEMOPT=X'02' to get more than about 15000-18000 or so actually allocated to NUMBUF. From V7.4 onwards, if the M204XSVC module has been linked into the nucleus, then XMEMOPT defaults to X'03', which includes the X'02' setting, and thus the MAXBUF setting should always 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:

M204.1190: MAXBUF SET TO SAME VALUE AS MINBUF -%C

You can use the [[MONITOR_command:_Disk_buffers|MONITOR DISKBUFF command to display, from each table of each file, the number of 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 Prefetchfeature.