Kevin Closson's Blog: Platforms, Databases and Storage

The blogging platform I use, WordPress, allows me to see from whence readers are being referred to my blog. I’ve gotten a couple of hits from this post on Jeff Browning’s blog. I’m not sure if that particular post is a critique or a caricature of my paper about Oracle Database 11g Direct NFS. Nonetheless, shortly after I first saw the post I privately informed Jeff of four inaccuracies contained in the post. They were:

1) Jeff’s assertion that Direct NFS will work on any NAS device is incorrect. I know of one NAS device in particular that DNFS will not function on whatsoever. Well, that’s not exactly true. It will function to the point of creating a database, but will corrupt the control file in doing so. I’m not going to state the particular manufacturer, because it is still unclear if there is a design incompatibility problem or simply a bug in that prevents that particular NAS device from functioning with DNFS.  Nonetheless, Jeff still hasn’t corrected the following quote, so I am here and now. Jeff said:

As such dNFS will work in exactly the same manner, with identical performance benefits, on any NAS device from any vendor.

That is incorrect.

2) Jeff uses incorrect terminology to explain one of the benefits of Direct NFS. That term is “context swapping”, and I quote:

Here is the theory behind dNFS. I/O on a database server occurs in a combination of user space and kernel space. Context swaps between the two spaces are expensive in terms of CPU cost. If you can move a part of that activity from kernel space into user space, you can save CPU cost due to reduced context swapping.

I’m not playing word games. Transitions from user to kernel mode are routinely, and inaccurately, referred to as context switching and while that is one of my pet peeves I suspect from the context of Jeff’s post that context switching is the term he actually meant. The problem is that system calls do not result in a context switch.

A context switch is the stopping of one process and switching to another for the sake of process scheduling. If a system call does not block, there is no context switch. An example would be getpid(2). On the other hand, a system call that does block (e.g., synchronous physical I/O) will count as a context switch because the process parked itself on a blocking system call. The scheduler will switch to a runable process according to such criteria as mode, priority, processor affinity and so on. The sum total of context switch categories is voluntary and involuntary. The former is when a process calls a blocking system call (or any other voluntary yield such as sched_yield(2)), the latter is when a user mode process executes to the end of its time slice at such a time as there is a runable process for the kernel to switch to.

And yes, I say all this about context switching versus context swapping with total disregard for this patent. In Unix/Linux, the term is context switch and knowing what that really means makes the output of monitoring commands such as vmstat(1) a little more meaningful I should think.

3) He stated 11g x86_64 was not available yet at the time of that blog post it was indeed available for download.

4) He misspelled my name in the blog post.

What is This Blog Post Really About?
Now, all of that aside, none of that is really what this blog post is about. This blog post is about a comment on Jeff’s blog. The reader posted a comment as follows:

One would assume that an OS vendor would be better at making NFS and multipathing work rather than a database vendor. Oracle has a disadvantage in that it has to test its client on all the various flavors of OS out there.

One would indeed assume just that-correctly in fact. Oracle Database 11g doesn’t make NFS better. Direct NFS is a replacement for NFS. While NFS is a great storage presentation model for Oracle (as I’ve said so many times), it does much more than Oracle requires. So, DNFS strips all that overhead out. Direct NFS is essentially an RPC shot straight at the NAS device-no file-related system calls (e.g., open(2), pread(2), io_submit(2), etc). And, oh, while I’m at it, I’ll point out that open(2), read(2) and io_submit(2) are system calls that do not result in a context switch (unless the read suffers a page cache miss or is O_DIRECT or raw(8)). But that is not what this blog post is about.