I was asked this good questions, and I thought I would pass on the answer.
WRTHRSH is defined in CSQ6LOGP and is defined as Number of output buffers to be filled before they are written to the active log data sets.
It has an effect in rare situations, and generally it is not relevant, so you do not need to tune it.
The WRTHSH does not have much value when doing high logging rates.
Consider a single application running in MQ, putting a 100MB message, WRTHRSH is 20.
No other applications are running .
If the log is not doing any IO, when WRTHRSH(20) log buffers are full then MQ start an IO to write the log buffers which are full.
During this IO another 50 log buffers fill up. So when the IO finishes it it sees there are more than WRTHRSH(20) log buffers - it starts the next IO and writes out the full log buffers.
That is the simplest case, it gets more complex when there are other transactions running which are doing commits or out of syncpoint requests.
Our application puts it's 100MB message.
Our second application puts a 1KB message and does a commit.
At this time 7 log buffers are full.
A commit requires a log force (IO) so the logger writes out the current full log buffers - 7 of them.
While this IO is happening our 100 MB putter is still filling the log buffers.
Transaction 3 gets a message and commits - and needs a log force.
The number of full log buffers is 20.
Transaction 4 puts a message and commits and needs a log force.
The number of full log buffers is currently 34
The IO completes and sees it needs to do another IO (due to log forces) so it starts another IO, and writes the currently full log buffers ( perhaps 60 of them by now)
During this IO we fill up 290 log buffers.
At the end of the IO we write the next 128.... (maximum number of pages per IO)
After this IO completes we write out the next 128 buffers etc
In this scenario WRTHSH was not used.
The outcome of this is that WRTHRSH is irrelevant when there are frequent commits ( log forces) happening. It really kicks in, for the
first IO when the system has been idle - so for many customers just once a day!