User's Manual

Debugging performance issues 10
Debugging performance issues
Improperly configured benchmark
Issue
The most common issue in achieving performance with the IO Accelerator is the failure to properly set up the
micro benchmark.
Solution
Be sure that you start with the benchmarks described in the previous sections to insure that the system is
performing properly with a known benchmark.
Oversubscribed bus
Issue
It is possible to install multiple IO Accelerators and other PCIe peripherals in a way that causes unequal
performance from each of the drives. In networking, this is frequently called over-subscription, and it is
common in all but the latest generation of systems. For PCIe bus based high performance peripherals, having
an over-subscribed topology can drastically affect performance, especially if the drives are set up for a RAID
configuration.
An example of a balanced topology, which maximizes performance, is a system that has 16 PCI lanes
connected from IO Accelerators to a switch chip, and 16 lanes from the switch chip to the root complex.
An example of an over-subscription condition, which decreases performance, is a system that has 12 PCI
lanes connected from IO Accelerators to a switch chip, but only 8 lanes from the switch chip to the root
complex.
Other PCIe devices, such as high performance network cards and graphic cards, can also create an
over-subscription condition.
Solution
To verify that there are no bandwidth bottlenecks in the PCIe bus, run the fio-pci-check utility and look
for errors.
NOTE: The fio-pci-check utility is not fully functional on all operating systems.
Issue
A related common performance problem that is harder to diagnose is when the PCIe lanes are run off of the
south bridge. Running off the south bridge is inherently slower than running off the north bridge. This can
create an oversubscribed configuration that the fio-pci-check utility might not be able to diagnose.
Solution
Diagnosing this issue is beyond the scope of this document. For assistance in determining if your system
suffers from this problem, run the fio-bugreport utility and the results of the system-vetting benchmarks.