Re: sorted writes for checkpoints

Jeff Janes wrote:

> Assuming the ordering is useful, the only way the OS can do as good a
> job as the checkpoint code can, is if the OS stores the entire
> checkpoint worth of data as dirty blocks and doesn't start writing
> until an fsync comes in.  This strikes me as a pathologically
> configured OS/FS.  (And would explain problems with fsyncs)
>   

This can be exactly the situation with ext3 on Linux, which I believe is 
one reason the write sorting patch didn't go anywhere last time it came 
up--that's certainly what I tested it on.  The slides for my talk 
"Righting Your Writes" are now up at 
http://projects.2ndquadrant.com/talks and an example showing this is on 
page 9.  I'm hoping to get the 3 patches shown in action or described in 
that talk submitted to the list before the next CommitFest.  You really 
need timing of individual sync calls to figure out what's going on here, 
and what happens is completely dependent on filesystem.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books