Friday, May 29, 2009

A couple of things about DB2 STMM and zLinux

It's been quite some time since I've posted to this blog. There are so many good resources for DB2 information today that I've been struggling to find a topic I felt was worthy of posting. Maybe, since I live in Cleveland the only thing 'worthy' of blogging about may be Lebron James. But, I'm writing once again on Linux or zLinux for two reasons. One, to share with you some information I've acquired while assisting customers with DB2 and second, to get your feedback on your experiences.

I thought I'd post a collaboration of the information I've used while researching a few issues that some customer's have been experiencing. All of these have had something to do with either (STMM) DB2's Self Tuning Memory Manager or Linux (with a focus on zLinux), or both.

First, lets start with some clarification on the limitation of DATABASE_MEMORY=AUTOMATIC on some Linux kernels.

1) Some older linux kernels had limitations in the capabilities for supplying adequate OS information for STMM. Thus, the database_memory setting reverts from automatic to computed on these platforms. If you are on SLES10 SP1 or RHEL5 or higher, then this limitation has been removed.
The details of this limitation are documented in the InfoCenter at:

OK, so your on one of the linux distributions and kernels that support automatic, but, you are still experiencing issues.
2) On Linux platforms there were several APARs involving STMM that were fixed in FP2 & FP3, so I'd recommend getting to FP3 if you are planning on using STMM on Linux.

Here are a few to note, but you can find all related APARs by searching for 'Linux+STMM' from the DB2 APAR Search.

3) Another common question or problem is the encounter of 'Unable to find donor' . This technote describes how and why that may be happening:

4) On zLinux, DIO (Direct I/O) is not the default setting. This is because the current support for zLinux requires SCSI over FCP due to alignment. This requirement is documented at :

In one of my older posts, I noted DIO as one of my top ten tuning tips.
If you are going to use STMM, I would recommend that you use DIO. Actually, DIO is a best practice even if your not using STMM. To learn more about how DB2 and the linux file system cache relate, this article on developerworks does as nice summary.

You can find additional tuning tips for zLinux at:

Lastly, if your using STMM on a zLinux platform I'd like to hear about your experiences, the good , the bad and the ugly. Especially if you are using or testing Collabrative Memory Management (CMM) for z/VM with DB2 9.5 STMM enabled. Comment or email me, I'm interested.