Kevin Closson's Blog: Platforms, Databases and Storage

Doug Burns has started an interesting blog thread about the Oracle Database 11g PARALLEL_IO_CAP_ENABLED parameter in his blog entry about Parallel Query and Oracle Database 11g. Doug is discussing Oracle’s new concept of built-in I/O subsystem calibration-a concept aimed at more auto-tuning database instances. The idea is that Oracle is trying to make PQ more aware of the down-wind I/O subsystem capability so that it doesn’t obliterate it with a flood of I/O. Yes, a kinder, gentler PQO.

I have to admit that I haven’t yet calibrated this calibration infrastructure. That is, I aim to measure the difference between what I know a given I/O subsystem is capable of and what DBMS_RESOURCE_MANAGER.CALIBRATE_IO thinks it is capable of. I’ll blog the findings of course.

In the meantime, I recommend you follow what Doug is up to.

A Really Boring Blog Entry
Nope, this is not just some look at that other cool blog over there post. At first glance I would hope that all the regular readers of my blog would wonder what value there is in throttling I/O all the way up in the database itself given the fact that there are several points at which I/O can/does get throttled downwind. For example, if the I/O is asynchronous, all operating systems have a maximum number of asynchronous I/O headers (the kernel structures used to track asynchronous I/Os) and other limiting factors on the number of outstanding asynchronous I/O requests. Likewise, SCSI kernel code is fit with queues of fixed depth and so forth. So why then is Oracle doing this up in the database? The answer is that Oracle can run on a wide variety of I/O subsystem architectures and not all of these are accessed via traditional I/O system calls. Consider Direct NFS for instance.

With Direct NFS you get disk I/O implemented via the remote procedure call interface (RPC). Basically, Oracle shoots the NFS commands directly at the NAS device as opposed to using the C library read/write routines on files in an NFS mount-which eventually filters down to the same thing anyway, but with more overhead. Indeed, there is throttling in the kernel for the servicing of RPC calls, as is the case with traditional disk I/O system calls, but I think you see the problem. Oracle is doing the heavy lifting that enables you to take advantage of a wide array of storage options-and not all of them are accessed with age-old traditional I/O libraries. And it’s not just DNFS. There is more coming down the pike, but I can’t talk about that stuff for several months given the gag order. If I could, it would be much easier for you to visualize the importance of DBMS_RESOURCE_MANAGER.CALIBRATE_IO. In the meantime, use your imagination. Think out of the box…way out of the box…