Updated: Sep 21, 2018
by David Hallberg
As Halloween approaches, I’ll look at a second tuning standard for AIX that has taken on the status of an urban legend. I stumbled on both last week during a client visit. Like last week’s phantom belief that you need to have AIX disk I/O pacing enabled to make Oracle run faster, this week’s myth involves Millennium® users running on IBM’s AIX operating system. Unlike last week, however, today’s myth can be traced directly back to me.
While working at Cerner in 1997, I stumbled on a tuning parameter Millennium was not using: the logical volume manager buffer count, or lvm_bufcnt. The parameter was designed to provide greater I/O throughput on raw devices, which for Millennium deployments applied to the nodes where the Oracle database was running. I began a campaign to get this parameter tuned.
During this time, Millennium ran with both the application and database on the same node and was only a 32-bit system. The 32-bit architecture severely limited the amount of memory on these nodes, yet lvm_bufcnt needed memory to buffer disk reads and writes. As a compromise, we determined that setting the lvm_bufcnt to 20 would provide better Oracle throughput without consuming too much memory.
Now that Millennium is a 64-bit system and its application nodes and database nodes are split, the tuning standard for this parameter is very different. But, at least for the client I visited last week, the setting has remained the same. What should the new parameters be?
For the application nodes, the setting should be 9, which is the default. Because there are no raw logical volumes (LVs) mounted on the application nodes, the higher setting of 20 is simply a waste of memory.
For the Oracle nodes, the setting should be 64, which is the maximum value. These databases benefit from having the highest amount of memory buffers for performing reads and writes to the raw LVs.
Many urban myths have a kernel of truth in them. In this case, the truth that existed years ago under very different operational conditions still haunts us. It seems like a perfect time for an exorcism.
Prognosis: Millennium users running on AIX can set the LVM buffer count on the application and database nodes for greater I/O throughput.