Oracle IOPS and HBA Queue Depth

About a month ago I wrote an overview of Linux Caching and I/O Queues as they pertain to Oracle. I was working on a project to architect, install and configure the beginnings of an 8-node cluster consisting of either one or two RAC databases. During the project, while I was waiting for the OS guys to resolve some networking issues, I ran a bunch of benchmarks on the storage subsystem. Specifically, I experimented with the size of the HBA Queue Depth to see if it would make a difference in performance.

But before getting into the results, a quick overview of our configuration: it was 11g RAC on Red Hat Enterprise Linux 5; Dell servers with four dual-core Opteron chips each. The RAC cluster initially had four nodes but will grow to at least eight as the data is migrated. The system has 4G QLogic cards, a McData switch and a 3Par SAN (which is blazing fast). ASM (no CFS) and dedicated Oracle Homes. The first spec had an InfiniBand interconnect but after a teleconference with Alex from Pythian discussing the project’s specific requirements, the spec was updated to use redundant Gigabit Ethernet.

Picking up where I left off: the default limit set by the Linux qla2xxx driver for concurrent I/O requests on QLogic cards (32 per LUN) is conservative. So can I increase performance by increasing this limit? The best way to answer a question like this is simply to try it.
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