DCMT VARY Commands › DCMT VARY JOURNAL › DCMT VARY JOURNAL Usage

DCMT VARY JOURNAL Usage

Forcing a Journal SWAP

The SWAP option of the DCMT VARY JOURNAL command directs the DC/UCF system to switch the active journal file from one disk journal file to another. If only one journal file is online and usable, the contents of the journal file must be offloaded before the command can complete and journaling resume. This causes a delay in the execution of all update transactions until the swap completes.

Varying a Specific Journal File

The DCMT VARY JOURNAL FILE command is intended for solving disk journal problems such as I/O errors while DC/UCF remains active. Before issuing any DCMT VARY JOURNAL FILE command, see Recovery Procedures from Journal File I/O Errors in the Database Administration Guide.

To successfully issue a VARY JOURNAL FILE command, the target journal file must not be the active journal file. Additionally, the following restrictions apply depending on the nature of the change:

• To vary the data set name of the journal file or to allocate or deallocate the file, it must be offline or inactive or have encountered an I/O error.
• To vary a journal file whose status is permanently offline to an active or online state, requires that change tracking be active.

Varying a Disk Journal File Offline

When varying a disk journal file offline, the system quiesces use of the journal file before marking it as offline. While the journal file is quiescing, the following is true:

• No further journal images are written to the journal file.
• The journal file remains available for recovery operations until all transactions having journal images on the disk journal have been terminated.
• The journal file remains in a pending offline state until all journal images contained on the file have been offloaded by an ARCHIVE JOURNAL utility statement.

Note: Once the journal file reaches the quiesced state, it is closed.

The DCMT DISPLAY JOURNAL FILE command is used to determine which transactions may have journal images on the target file.

Dynamically Allocating and Deallocating Journal Files

The ability to dynamically allocate and deallocate journal files is operating system and file-type dependent. The restrictions are the same as those for database files.

What the Fragment Interval Does

The fragment interval is the number of journal blocks to be written to the journal file before CA IDMS writes a dummy segment (DSEG) record to the journal file. DC/UCF uses the DSEG records in the event of a system crash to determine the appropriate starting place for warmstart processing. Recovery processing begins at the most recently accessed journal fragment.

The fragment interval is a value between 0 and the highest relative block number (RBN). You can determine the highest RBN with a DCMT DISPLAY JOURNALS command. Because of overhead involved in writing dummy journal records, a value of at least 100 is recommended.

Assigning a Journal Transaction Level

The journal transaction option allows you to reduce journal I/O. It is most effective when several programs are updating the database concurrently.

When you establish a journal transaction level, CA IDMS defers writing a journal buffer page to the journal when your transaction issues a COMMIT, ROLLBACK, or FINISH statement until either of the following occurs:

• The journal buffer page is filled by other transactions.
• The number of transactions writing to the journal falls below the value specified for the journal transaction level.

Your transaction is dispatched when the journal I/O is completed.

Note: If the transaction level you specify is too low, the number of active transactions may never be low enough to initiate a journal I/O; only a full buffer initiates a journal I/O.