Controlling Log Data Set Characteristics

From time to time, you need to tune and modify log data set characteristics, for example, in the following circumstances:

• After monitoring

• After changing your requirements for system availability, integrity, or operator handling

For summaries of actions required to change log data set design, see:

• "Controlling the Online Log Data Set"

• "Controlling the Write-Ahead Data Set" on page 434

• "Controlling the System Log Data Set" on page 435

• "Controlling the RECON Data Sets" on page 435

Controlling the Online Log Data Set

Because you can restart IMS (warm start or emergency restart) with all input in an SLDS, you can reallocate the OLDSs between a shutdown (or failure) and a subsequent restart. To restart IMS using SLDSs as input, you must delete the PRIOLDS and SECOLDS records from the RECON data sets.

Table 24-11 on page 432 lists the actions required to change OLDS characteristics.

Table 24-11. Changing OLDS Characteristics

Modification	Actions Required
BLKSIZE[a]	Shut down IMS. Archive all OLDSs. Delete PRIOLDS and SECOLDS records from the RECON data sets using the DELETE.LOG command. Scratch all OLDSs. Reallocate OLDSs with the new BLKSIZE. Verify WADS space allocation. Restart IMS (from SLDS).
Single to dual[b]	Shut down IMS. Archive all OLDSs. Allocate dual OLDSs. Delete the OLDS records from the RECON data sets, using the DELETE.LOG command. The primary OLDS records are deleted. Change OLDSDEF specification to dual. Change IMS startup procedure (OLDS DD statements if required. Compile DFSMDA macros, if required. Modify operating procedures. Restart IMS.
Dual to single[b]	Shut down IMS. Archive all OLDSs. Delete OLDS record from the RECON data sets, using the DELETE.LOG command. The primary and secondary OLDS records are deleted. Delete secondary OLDSs. Change IMS startup procedure. Modify operating procedures. Restart IMS.
BUFNO[c]	Shut down IMS. Change the OLDSDEF specification for BUFNO. Verify CSA size. Verify WADS space allocation. Restart IMS.
Space, location, or allocation[d]	Shut down IMS. Archive all OLDSs. Delete PRIOLDS and SECOLDS records in the RECON data sets, using the DELETE.LOG command. Scratch and reallocate OLDSs. Restart IMS (from SLDS).

^[a] Changing OLDS Block Size: Changing OLDS block size affects WADS space allocation. For information on how to calculate WADS space requirements, see IMS Version 9: Installation Volume 1: Installation Verification. To change WADS space allocation, see Table 24-12 on page 434. All OLDSs must have the same block size.

^[b] Changing the Mode: You must change your OLDSDEF specification in the DFSVSMxx member in IMS.PROCLIB. IMS initialization requires that at least three pairs of OLDSs be available. You must also reconsider data set placement.

When changing from single to dual OLDS, each data set in a pair of OLDSs must have the same space allocation (number of blocks).

Changing the mode from single to dual or from dual to single requires changes in the following operating procedures:

• Skeletal JCL for the Log Archive utility (ARCHJCL member)

• Skeletal JCL for the Log Recovery utility (LOGCLJCL member)

• Batch JCL for Log Recovery utility

• Batch backout JCL for online transactions and BMPs

If you warm start the IMS subsystem after changing from single to dual OLDSs, the /DISPLAY OLDS command does not show the secondary OLDSs as IN USE until they have been archived once. The command does, however, show dual OLDS logging.

^[c] Changing the Number of OLDS Buffers: Change the BUFNO keyword on the OLDSDEF statement in the DFSVSMxx member in IMS.PROCLIB.

When you modify the BUFNO keyword, you should consider also modifying the region size for the VSAM common segment area (CSA). The amount of storage fixed for OLDS buffers is BUFNO * BUFFERSIZE.

WADS space is also affected by the value specified on the BUFNO keyword. To change WADS space allocation, see Table 24-12 on page 434.

Related Reading: For information on how to calculate WADS space requirements, see IMS Version 9: Installation Volume 1: Installation Verification.

^[d] Changing Space, Location, or Allocation: For the recommended method of changing the space or location of your OLDS, or for reallocating an OLDS on the same volume and with the same space, follow the procedure for changing the BLKSIZE in note a.

You can modify space, location, or allocation without shutting down IMS in a non-XRF environment by using the following procedure:

1. /STOP OLDS nn

2. Archive all OLDSs.

3. Delete PRIOLDS and SECOLDS records in the RECON data sets, using the DELETE.LOG command.

4. Scratch and reallocate OLDSs.

5. /START OLDS nn

Using Newly Initialized Volumes for OLDS

If a newly initialized (or reinitialized, but unformatted) volume is to contain an OLDS, you must format either the volume or the space occupied by the OLDS before the online system uses it. If you do not format the volume, or if the block size of the new OLDS data set is not the same as the existing OLDS data set, you can expect severe performance degradation and excessive device and channel usage until IMS completely fills the OLDS once. This problem is especially noticeable during emergency restart and during XRF tracking and takeover.

You can use any of the following techniques to format a volume for an OLDS:

• Copy an existing OLDS (of the same size) into the new OLDS.

• Copy an existing volume into the new volume, scratch the volume table of contents (VTOC), and allocate the new OLDS.

• Use another IMS subsystem to fill the OLDS: turn on all traces to the log and issue checkpoint commands until the OLDS fills.

• Write a program that either writes at least one byte of data into each track on the volume or fills the OLDS with maximum logical record length (LRECL) blocks.

Controlling the Write-Ahead Data Set

Table 24-12 lists the actions required to change WADS characteristics.

Table 24-12. Changing WADS Characteristics

Modification	Actions Required
Single to dual[a]	Shut down IMS. Allocate new WADS. Define a DFSMDA member. Add DD statement in IMS JCL, if necessary. Code WADS=D in IMS JCL. Modify operating procedures. Restart IMS with FORMAT WADS keywords.
Dual to single[a]	Shut down IMS. Code WADS=S in IMS JCL. Delete DFSMDA member. Remove DD statement in IMS JCL, if necessary. Modify operating procedures. Restart IMS.
Adding spare	Allocate a spare WADS. Update WADSDEF specification in the DFSVSMxx member of IMS.PROCLIB. Define DFSMDA member. Add DD statement in IMS JCL, if necessary. Modify operating procedures. /START WADS n (or wait until IMS restart).
Removing spare	/STOP WADS n (and wait for dynamic deallocation). Scratch spare WADS. Update WADSDEF statement in the DFSVSMxx member of IMS.PROCLIB. Remove DD statement in IMS JCL. Modify operating procedures.
Space, location, or allocation[b]	Shut down IMS. Scratch and reallocate WADSs. Restart IMS with FORMAT WADS keywords.

^[a] Changing the Mode: WADS can be dynamically allocated and deallocated.

To reflect the new mode for WADSs, you must update the:

• Skeletal JCL for the Log Recovery utility (LOGCLJCL member).

• IMS startup procedure (WADS= execution parameter).

• All recovery procedures implemented to recover WADS errors or to close unclosed OLDSs using WADS.

^[b] Changing the Space, Location, or Allocation: All WADS must have the same space allocation (number of tracks) and be on the same type of device.

Controlling the System Log Data Set

Converting from single to dual SLDSs requires modification in the skeletal JCL (ARCHJCL member) and in all your operational procedures that use SLDSs. In the operational procedures, consider using the secondary SLDS when you experience errors in the primary SLDS. No modification is required for online processing because IMS dynamically allocates SLDSs.

Changing the BLKSIZE requires modification in the skeletal JCL (ARCHJCL member). All SLDSs required for online processing must have the same BLKSIZE.

Controlling the RECON Data Sets

Table 24-13 on page 436 lists the actions required to change RECON data set characteristics.

Table 24-13. Changing RECON Data Set Characteristics

Modification	Actions Required
Adding spare	Define a spare RECON data set. Compile DFSMDA macro for the spare data set or add DD statement in IMS JCL and batch JCL if you do not use dynamic allocation. The spare data set must be in VSAM CREATE mode.
Removing spare	Delete cluster. Delete DFSMDA member in IMS.SDFSRESL or remove DD statement from IMS JCL and batch JCL if you do not use dynamic allocation.
Replacing active	Recommendation: Stop all IMS subsystems and batch jobs that access the RECON data sets. Define a spare data set with new space or allocate a spare data set at a new location. CHANGE.RECON REPLACE (RECON n) Define a new spare data set. Continue normal processing.
Single to dual	Recommendation: Stop all IMS subsystems and batch jobs that access the RECON data sets. Define a spare RECON data set. CHANGE.RECON DUAL Define a new spare data set. Continue normal processing.

Recommendation:

For both online and batch, use dynamic allocation for RECON data sets, and run with at least three RECON data sets.