Re: Proposed LogWriter Scheme, WAS: Potential Large

On Mon, 2002-10-07 at 10:38, Antti Haapala wrote:
> Browsed web and came across this piece of text regarding a Linux-KAIO
> patch by Silicon Graphics...
>

Ya, I have read this before. The problem here is that I'm not aware of
which AIO implementation on Linux is the forerunner nor do I have any
idea how it's implementation or performance details defer from that of
other implementations on other platforms. I know there are at least two
aio efforts underway for Linux. There could yet be others. Attempting
to cite specifics that only pertain to Linux and then, only with a
specific implementation which may or may not be in general use is
questionable. Because of this I simply left it as saying that I believe
my analysis is pessimistic.

Anyone have any idea of Red Hat's Advanced Server uses KAIO or what?

>
> Preliminary experience with KAIO have shown over 35% improvement in
> database performance tests. Unit tests (which only perform I/O) using KAIO
> and Raw I/O have been successful in achieving 93% saturation with 12 disks
> hung off 2 X 40 MB/s Ultra-Wide SCSI channels. We believe that these
> encouraging results are a direct result of implementing a significant
> part of KAIO in the kernel using split-phase I/O while avoiding or
> minimizing the use of any globally contented locks."

The problem here is, I have no idea what they are comparing to (worse
case read/writes which we know PostgreSQL *mostly* isn't suffering
from). If we assume that PostgreSQL's read/write operations are
somewhat optimized (as it currently sounds like they are), I'd seriously
doubt we'd see that big of a difference. On the other hand, I'm hoping
that if an aio postgresql implementation does get done we'll see
something like a 5%-10% performance boost. Even still, I have nothing
to pin that on other than hope. If we do see a notable performance
increase for Linux, I have no idea what it will do for other platforms.

Then, there are all of the issues that Tom brought up about
bloat/uglification and maintainability. So, while I certainly do keep
those remarks in my mind, I think it's best to simply encourage the
effort (or something like it) and help determine where we really sit by
means of empirical evidence.

Greg