* Greg Stark <greg.stark@enterprisedb.com> [081024 10:48]:
> I thought about how to support both and ran into probblems that would
> make the resulting solutions quite complex.
>
> In the libaio view of the world you initiate io and either get a
> callback or call another syscall to test if it's complete. Either
> approach has problems for postgres. If the process that initiated io
> is in the middle of a long query it might takr a long time ot even
> never get back to complete the io. The callbacks use threads...
>
> And polling for completion has the problem that another process could
> be waiting on the io and can't issue a read as long as the first
> process has the buffer locked and io in progress. I think aio makes a
> lot more sense if you're using threads so you can start a thread to
> wait for the io to complete.
>
> Actually I think it might be doable with a lot of work but I'm worried
> that it would be a lot of extra complexity even when you're not using
> it. The current patch doesn't change anything when you're not using it
> and actually is quite simple.
In the Solaris async IO, are you bound by direct IO? Does the OS page-cache
still get primed by async reads? If so, how about starting async IO
into a "throwaway" local buffer; treat async IO in the same way as
fadvise, a "pre-load the OS page cache so the real read is quick".
Sure, I understand it's not the "perfect model", but it I don't see
PostgreSQL being refactored enough to have a pure async model happening
any time in the near future...
--
Aidan Van Dyk Create like a god,
aidan@highrise.ca command like a king,
http://www.highrise.ca/ work like a slave.
