NJBUFF parameter

Number of journal buffers

Summary

Default value

The default for z/OS and z/VM is NSERVS + NSUBTKS + 1. The default for z/VSE is 1, because z/VSE does not support multiple journal buffers.

Parameter type

System

Where set

Cannot be reset

In z/OS, the NJBUFF parameter can be set explicitly only via the PARM field in the EXEC card. See Specifying EXEC statement parameters.

In z/VM, the NJBUFF parameter is stacked in the EXEC that issues all the FILEDEFs.

Related products

All

Introduced

Model 204 V2.1

Description

The NJBUFF parameter has a dual role of allocating the number of channel programs and journal buffers.

NJBUFF can be 1, or NSERVS + NSUBTKS + 1, or any value larger than NSERVS + NSUBTKS + 1. Setting NJBUFF to a value greater than 1 invokes multiple journal buffers and ensures that a free buffer is always available for the journal.

In z/OS only, the NJBUFF setting is automatically recalculated when the setting is greater than 1 and less than or equal to NSERVS + NSUBTKS + 1. The initial value of NJBUFF is used to define the number of channel programs (NCP) for the journal data set. Sizing NJBUFF to the value calculated in the formula guarantees the optimal number of NCP for the journal data set.

The NJBUFF parameter has the following options:

NJBUFF parameter options

NJBUFF=	Option
0 or is omitted	The default value is used.
1	Only one buffer and one channel program (NCP) is obtained. All puts and waits are synchronized. Recommended only when NUSER=1.
Greater than NSERVS + NSUBTKS + 1	The specified number of buffers is obtained. If the value is greater than 255 and 31-bit BSAM is allowed, only 255 NCPs are obtained. If 31-bit BSAM is not allowed, only 99 NCPs are obtained. If the buffer number is larger than the NCPs, for example, NJBUFF=500, then 500 buffers are obtained, but only a maximum of NCP buffers are active. I/Os are checked only when they reach the NCP limit or when they are forced.
For z/OS only: Greater than one and less than or equal to NSERVS + NSUBTKS + 1.	The value is recalculated by Model 204 as: NSERVS + NSUBTKS + 1 Message M204.0434 is issued.