Re: Load distributed checkpoint

On Fri, 8 Dec 2006, Takayuki Tsunakawa wrote:

> Though I'm not sure, isn't it the key to use O_SYNC so that write()s
> transfer data to disk?

If disk writes near checkpoint time aren't happening fast enough now, I
doubt forcing a sync after every write will make that better.

> If PostgreSQL is to use these, writing of dirty buffers has to be
> improved. To decrease the count of I/O, pages adjacent on disk that
> are also adjacent on memory must be written with one write().

Sorting out which pages are next to one another on disk is one of the jobs
the file system cache does; bypassing it will then make all that
complicated sorting logic the job of the database engine. And unlike the
operating system, the engine doesn't even really know anything about the
filesystem or physical disks involved, so what are the odds it's going to
do a better job? That's the concern with assuming direct writes are the
solution here--you have to be smarter than the OS is for that to be an
improvement, and you have a lot less information to make your decisions
with than it does.

I like Itagaki's idea of some automatic tuning of the checkpoint timeout
and wal segment parameters to help out with checkpoint behavior; that's an
idea that would help a lot of people. I'm not so sure that trying to make
PostgreSQL take over operations that it's relying on the OS to handle
intelligently right now will be as helpful, and it's a big programming job
to even try.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD