SET commands in DB2 for z/OS

SET versus SQL SET

Beginners mix these up constantly. SET CURRENT SCHEMA is SQL. -SET LOG SUSPEND is an operator command you type with the subsystem prefix from SDSF, a DSN session, DB2I COMMANDS, IMS/CICS, or IFI. Abbreviations: -SET ARC for SET ARCHIVE. All three SET commands have member data sharing scope: they affect the member whose prefix you used.

-SET ARCHIVE

-SET ARCHIVE sets the maximum number of tape units dedicated to reading archive logs, and how long an allocated archive-read tape may sit idle before z/OS deallocates it. It overrides the installation values (READ TAPE UNITS / COUNT and DEALLOC PERIOD / TIME) until Db2 restarts. Authorization: ARCHIVE privilege, SYSOPR, SYSCTRL, or SYSADM.

COUNT(integer)

Maximum tape units for concurrent archive reads: 1 to 99. Raising COUNT allows more concurrent unique archive tape data sets. Lowering COUNT deallocates inactive units immediately; tapes that are active or premounted stay allocated until they become inactive. If the operator replies CANCEL to IEF238D (device name or cancel), COUNT is reset to the number of units already obtained.

TIME

• TIME(minutes) — 0 to 1439 minutes of idle time before deallocation
• TIME(,seconds) — 1 to 59 seconds (the comma form is how you specify seconds only, for example TIME(,30))
• TIME(NOLIMIT) or TIME(1440) — never deallocate for idle time; seconds are not allowed with 1440

DEFAULT

Restores COUNT and TIME to the values specified during Db2 installation.

For best archive-read performance, IBM suggests the largest COUNT and TIME your tape configuration can support. In data sharing, a tape kept allocated on one member is unavailable to other members. If recover jobs run on different members, you often want DEALLOC PERIOD of 0 and you may avoid a large COUNT so tapes free quickly.

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-SET ARCHIVE COUNT(2) TIME(,30)
-SET ARCHIVE COUNT(4) TIME(2)
-SET ARCHIVE COUNT(1) TIME(1440)
-SET ARCHIVE DEFAULT

Responses include messages in the DSNJ334I–DSNJ337I range confirming the new count and time.

-SET LOG

-SET LOG changes checkpoint frequency, suspends or resumes logging and updates, and can add (or, in current releases, remove) an active log data set. Checkpoint and suspend/resume changes are temporary across restart (LOGLOAD takes effect after the next system checkpoint). NEWLOG / REMOVELOG changes are pervasive because they update the BSDS. Same ARCHIVE / SYSOPR / SYSCTRL / SYSADM authorization as SET ARCHIVE.

SINGLE, BOTH, LOGLOAD, and CHKTIME

• SINGLE — only LOGLOAD or only CHKTIME controls checkpoints. If you specify SINGLE you must also specify LOGLOAD or CHKTIME. Switching from BOTH to SINGLE changes the mode.
• BOTH — both thresholds are armed; whichever is reached first takes a checkpoint and resets both. If you omit LOGLOAD or CHKTIME, the existing value (or the default) is used. Defaults when a value was never set: CHKTIME 5 minutes, LOGLOAD 500000 log records.
• LOGLOAD(integer) — log records between the start of successive checkpoints. 0 requests a checkpoint now without changing the interval. Ranges: 1000–16000000 when SINGLE is explicit; 0 or 1000–16000000 when SINGLE mode is already in effect; 0 or 1000–99999999 when BOTH is in effect.
• CHKTIME(integer) — minutes between checkpoint starts. 0 means checkpoint now. 1–1439 when SINGLE is explicit; 0–1439 otherwise.

A very large interval can make restart after an abend slow because Db2 must process more log. A very small interval causes excessive checkpoints. LOGLOAD(0) / CHKTIME(0) is synchronous from a batch job and asynchronous from a console. In data sharing, avoid requesting that immediate checkpoint while any member has SET LOG SUSPEND—the checkpoint can wait until RESUME. DISPLAY LOG shows whether CHKTIME, LOGLOAD, or both are in use. After restart, checkpoint values return to the subsystem parameter (or whatever SET SYSPARM last loaded).

SUSPEND and RESUME

SUSPEND externalizes unwritten log buffers, takes a system checkpoint in non-data-sharing, updates the BSDS with the high-written RBA, then suspends update activity. Message DSNJ372I stays on the console until resume. Writes of 32 KB pages and data set extensions for all page sizes are quiesced so a volume copy does not capture a torn 32 KB page or a VSAM catalog that disagrees with the data set. Read-only activity can continue. Update activity stays suspended until SET LOG RESUME or STOP DB2.

SUSPEND is not allowed when ARCHIVE LOG or STOP DB2 is already running a system quiesce. While suspended, do not issue ARCHIVE LOG unless you also specify CANCEL OFFLOAD. Do not keep logging suspended during high activity or for a long time—lock timeouts and diagnostic dumps are common side effects. The copy taken between SUSPEND and RESUME can contain uncommitted data; restoring that copy and restarting Db2 lets restart recovery make the subsystem consistent.

Issue RESUME from a z/OS console or the installation SYSADM ID. While the log-write latch is held, command-authorization checking from other paths can hang until logging resumes. In data sharing: SUSPEND on one member, wait until it completes, then SUSPEND the remaining members.

NEWLOG, COPY, and REMOVELOG

NEWLOG(data-set-name) adds a newly defined active log to the BSDS inventory if Db2 can open it. It is available immediately—no recycle. Define the VSAM linear data set with IDCAMS first. IBM recommends formatting with DSNJLOGF before NEWLOG. COPY(1|2) identifies copy 1 or copy 2; in dual logging, add both copies. Current Db2 releases also support REMOVELOG to delete an active log data set from the BSDS when you have drained that log and no longer need it in the inventory.

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-SET LOG LOGLOAD(0)
-SET LOG LOGLOAD(150000)
-SET LOG BOTH CHKTIME(10) LOGLOAD(500000)
-SET LOG SINGLE LOGLOAD(500000)
-SET LOG SUSPEND
-SET LOG RESUME
-SET LOG NEWLOG(DSNC910.LOGCOPY1.DS04) COPY(1)
-SET LOG NEWLOG(DSNC910.LOGCOPY2.DS04) COPY(2)

-SET SYSPARM

-SET SYSPARM loads a subsystem parameter (ZPARM) module while Db2 is up, changing parameters that are defined as online updatable. Member scope.

• LOAD(load-module-name) — load that module into storage. The default name is DSNZPARM. The module is the output of installation job DSNTIJUZ.
• RELOAD — reload the last named module (pick up a relinked copy of the same name).
• STARTUP — reset parameters to the values this member had at Db2 startup, discarding later LOAD/RELOAD changes.

Typical workflow: DSNTINST in Update mode, assemble/link the new module, ensure LLA has refreshed if the data set is in the LNKLST (F LLA,REFRESH after compressing a library), then SET SYSPARM. Do not issue SET SYSPARM while ARM is compressing the data set that contains the new module. If you are not authorized for installation SYSADM/SECADM parameters, Db2 keeps the previous values for those keywords and issues DSNZ015I for each unauthorized attempt.

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-SET SYSPARM LOAD(ADMPARM1)
-SET SYSPARM RELOAD
-SET SYSPARM STARTUP

Operator habits that prevent outages

• DISPLAY LOG before and after SET LOG so you can see current checkpoint mode, suspend state, and log copy status
• DISPLAY ARCHIVE after SET ARCHIVE to confirm tape count and deallocation time
• Treat SUSPEND as a timed window with a named operator who will issue RESUME—never walk away
• Document every SET SYSPARM LOAD in the change record so the next IPL is not a surprise (startup values come from the module named in the MSTR JCL, not from the last LOAD unless you also updated the startup procedure)
• Remember SET commands are not SQL: they do not run under a plan, they do not commit a unit of recovery, and they do not belong in SPUFI

Explain It Like I'm Five

SET ARCHIVE is telling the librarian how many tape cassette players may stay plugged in, and how long a player can sit unused before you unplug it. SET LOG is adjusting how often Db2 writes a bookmark in its diary (checkpoints), or pressing pause on the diary so you can photocopy every page without a page turning mid-copy. SET SYSPARM is swapping the rule book Db2 is using while the library stays open—but only the pages that are allowed to change without closing the building.

Exercises

1. Write SET ARCHIVE to allow three tape units that deallocate after 60 seconds idle. Then write the command that puts COUNT and TIME back to ZPARM.
2. Your restart after a cancel is too slow. Which SET LOG options would you inspect with DISPLAY LOG, and what is the risk of setting LOGLOAD very low?
3. List the steps (IDCAMS, DSNJLOGF, SET LOG) to add dual-copy active logs DS08 copy 1 and copy 2 while Db2 is up.
4. Explain why SET SYSPARM LOAD(NEWMSTR) before a weekend does not automatically become the module used after next week's IPL.
5. In a three-member data sharing group, describe the safe order for SET LOG SUSPEND and why you would not suspend all members in one blast.