Re: Potential Large Performance Gain in WAL synching

Curtis Faith wrote:
> Bruce Momjian wrote:
> > I may be missing something here, but other backends don't block while
> > one writes to WAL.
> 
> I don't think they'll block until they get to the fsync or XLogWrite
> call while another transaction is fsync'ing.
> 
> I'm no Unix filesystem expert but I don't see how the OS can
> handle multiple writes and fsyncs to the same file descriptors without
> blocking other processes from writing at the same time. It may be that
> there are some clever data structures they use but I've not seen huge
> praise for most of the file systems. A well written file system could
> minimize this contention but I'll bet it's there with most of the ones
> that PostgreSQL most commonly runs on.
> 
> I'll have to write a test and see if there really is a problem.

Yes, I can see some contention, but what does aio solve?

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
359-1001+  If your life is a hard drive,     |  13 Roberts Road +  Christ can be your backup.        |  Newtown Square,
Pennsylvania19073