Re: checkpoint writeback via sync_file_range

On Wednesday, January 11, 2012 10:33:47 AM Florian Weimer wrote:
> * Greg Smith:
> > One idea I was thinking about here was building a little hash table
> > inside of the fsync absorb code, tracking how many absorb operations
> > have happened for whatever the most popular relation files are.  The
> > idea is that we might say "use sync_file_range every time <N> calls
> > for a relation have come in", just to keep from ever accumulating too
> > many writes to any one file before trying to nudge some of it out of
> > there. The bat that keeps hitting me in the head here is that right
> > now, a single fsync might have a full 1GB of writes to flush out,
> > perhaps because it extended a table and then write more than that to
> > it.  And in everything but a SSD or giant SAN cache situation, 1GB of
> > I/O is just too much to fsync at a time without the OS choking a
> > little on it.
> 
> Isn't this pretty much like tuning vm.dirty_bytes?  We generally set it
> to pretty low values, and seems to help to smoothen the checkpoints.
If done correctly/way much more invasive you could only issue sync_file_range's 
to the areas of the file where checkpointing needs to happen and you could 
leave out e.g. hint bit only changes. Which could help to reduce the cost of 
checkpoints.

Andres