I have a Solaris 10 x86_64 system running MySQL 5.5. Under heavy usage times, we're getting very slow responses from the database: slow queries running into minutes that normally return in sub-second times. CPU utilization is in the 60-70% range. Load average regularly gets well into the 20s, infrequently into the 40s, and I've seen it up to the 50s. (Two four-core CPUs with HyperThreading enabled.) It acts like an I/O problem, as if it's waiting on disks to write, but I'm not seeing any indications that there is any actual I/O problem. Average disk wait times are consistently 0, average wait queues are in the 0.2-0.3 range, and disk busy percentages occasionally creep into the 15% area. (All of this as according to sar.)
The storage is a zfs zpool of 5 zdev mirrors of two SAS drives. I do not have an intent log device, but I don't see that as being an issue with this workload.
What am I missing?