How can I increase the I/O queue depth to maximize performance

Question

Disk benchmarks typically have a chart that shows the I/O throughput increasing as the queue depth increases. I'm assuming that what's going on at the hardware level is that the disk controller is reordering the requests to coincide with where the magnetic head happens to be. But how can I increase the queue depth in my own I/O-heavy application? I'll be using C#, but language-agnostic answers will probably also be helpful.

One (untested) method that occurred to me was to split my disk access into many files and issue the I/Os in many threads. But surely there is a way to do it without using a thread per parallel I/O. And I'm also assuming that splitting to many files is not needed since a DB is generally one file.

Answer 1

You don't need to use a thread per I/O operation, but you do need to parallelize your I/O. Servers that perform large-scale I/O (like databases) typically use I/O completion ports (IOCP). Completion ports allow multiple I/O requests to be pending in parallel - and can maximize the underlying hardware's ability to reorder queued requests. IOCP use callbacks to a thread-pool to notify an application when pending I/O operations have completed.

Unfortunately, C# has limited (built-in) support for completion ports - however, there are some open-source libraries that attempt to bridge this gap.

Beware, IOCP is not for the weak of heart - and the performance benefit only accrues if you are truly performing a massive amount of I/O.