Frequently asked questions about journaling and disk unit usage

How many units in my disk pool will journaling use?

Starting in IBM® i 7.1, journal receivers will be spread across all disk units in the disk pool. Journaling no longer directs writes to specific disk units.

The journal receiver threshold value will influence the number of parallel writes that journal will allow. The higher the journal receiver threshold value, the more parallel I/0 requests will be allowed. Allowing more parallel I/O requests may improve performance.

For more information about disk unit use and journaling see: Striving for Optimal Journal Performance on DB2® Universal Database for iSeries (3.1 MB)

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Which journal parameters and settings affect the number of the disk units the journal receiver uses?

The threshold for the journal receiver and whether you use a maximum receiver-size option have the largest effect on how many disk units the journal receiver uses. If you have a system which is before V5R2, removing internal entries also affects the number of disk units that are used.

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Why is the system not using the new disk units I added to my disk pool?

There can be a several reasons. First, to use the newly added disk units, you must perform a change journal operation to attach a new journal receiver. Also, the system does not necessarily use all of the disk units in a disk pool. If you are not using a maximum receiver-size option, the most disk units the system will spread the receiver over is ten. The number of disk units the receiver uses also depends on the threshold you use for your journal receiver. If you use a maximum receiver-size option and increase your threshold, it is more likely that your new disk unit will be used.

If you use system-managed access-path protection (SMAPP), the system generates internal journal entries to protect the access paths for database files. If you have not upgraded to at least V5R2, setting your journal receiver to remove internal entries is an issue if you are not producing these internal entries. Before V5R2, removing internal entries can steal disk units from the normal journal entries. For example, if you have six disk units in the disk pool housing your journal receiver and remove internal entries, two units are dedicated to the internal entries and four units are used for your regular journal entries. If you do not produce any internal entries, those two units remained idle. For V5R2 and later, this is not an issue.

For more information about disk unit use and journaling see Striving for Optimal Journal Performance on DB2 Universal Database for iSeries.

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Why are some disk units used by journal receivers noticeably busier than the others and what can I do to spread out the usage?

The journal receivers probably use some disk units more than other because of the way journal management writes journal entries to disk. When the system produces journal entries, journal management stores the journal entries in memory. When it is ready, journal management sends the journal entries to a disk unit in one group. When the next group of journal entries are ready, journal management sends the entries to the next disk unit. Journal management continues in this sequential manner until all of the disk units it uses have received a group of journal entries. The cycle then repeats.

You can spread out the usage by increasing your threshold and using a maximum receiver-size option.

For more information about disk unit use and journaling see Striving for Optimal Journal Performance on DB2 Universal Database for iSeries.

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