I wrote:
> That said, if it's typical for many changes to made to a page
> internally before PG needs to commit that page to disk, then your
> argument makes sense, and that's especially true if we simply cannot
> have the page written to disk in a partially-modified state (something
> I can easily see being an issue for the WAL -- would the same hold
> true of the index/data files?).
Also, even if multiple changes would be made to the page, with the
page being valid for a disk write only after all such changes are
made, the use of mmap() (in conjunction with an internal buffer that
would then be copied to the mmap()ed memory space at the appropriate
time) would potentially save a system call over the use of write()
(even if write() were used to write out multiple pages). However,
there is so much lower-hanging fruit than this that an mmap()
implementation almost certainly isn't worth pursuing for this alone.
So: it seems to me that mmap() is worth pursuing only if most internal
buffers tend to be written to only once or if it's acceptable for a
partially modified data/index page to be written to disk (which I
suppose could be true for data/index pages in the face of a rock-solid
WAL).
--
Kevin Brown kevin@sysexperts.com