Basics on Journal Cache

Journal caching is a feature that per journal you can allow the system to bundle or cache journal entries in main storage before writing them to disk. Journal caching modifies the behavior of traditional non-cached journaling in batch. Without journal caching, a batch job waits for each new journal entry to be written to disk. Journal caching allows most operations to no longer be held up waiting for synchronous disk writes to the journal receiver. Batch applications which perform large numbers of changes to the data portion of the journaled objects often see the most benefit of journal caching. Changes can be made to files, data areas, data queues, or IFS objects. Applications that use commitment control will see less of a performance improvement because the commit operation will force the entries to disk, regardless of how many entries are bundled.

Journal caching is Option 42 of the i5/OS operating system:

57XXSS1 Opt. 42 HA Journal Performance

The JRNCACHE parameter on the Create Journal (CRTJRN) or Change Journal (CHGJRN) commands can be set to*NO or *YES to turn this function off and on per journal.

It is not recommended to use journal caching if it is unacceptable to lose even one recent change, as in the event of a system failure, where the contents of main memory are not preserved. This type of journaling is directed primarily toward batch jobs and may not be suitable for interactive applications were single system recovery is the primary reason for using journaling.

Furthermore, the results from the following commands or API will not display the journal entries in the cache:

o Display Journal (DSPJRN) command
o Retrieve Journal Entry (RTVJRNE) command
o Receive Journal Entry (RCVJRNE) command
o Retrieve Journal Entries (QjoRetrieveJournalEntries) API

The Display Journal Receiver Attributes (DSPJRNRCVA) command and the Retrieve Journal Receiver Information (QjoRtvJrnReceiverInformation) API show the total number of journal entries in a journal receiver. The DSPJRN, RTVJRNE, RCVJRNE commands or the QjoRetrieveJournalEntries API will not display the entries in the journal cache. For example, if there are 100 journal entries in a journal receiver, the DSPJRNRCVA command and QjoRtvJrnReceiverInvformation API will show the total number of entries is 100. However, if the last 25 entries are in the journal cache, you can only view the first 75 entries.

Journal caching also affects remote journaling. Journal entries are not sent to the remote system until they are written from the cache to disk. Because journal entries are not sent to the target system right away, the number of journal entries that are not confirmed are always greater than if you are not using journal caching.