Re: checkpointer continuous flushing

Hi,

I just pushed the two major remaining patches in this thread. Let's see
what the buildfarm has to say; I'd not be surprised if there's some
lingering portability problem in the flushing code.

There's one remaining issue we definitely want to resolve before the
next release:  Right now we always use one writeback context across all
tablespaces in a checkpoint, but Fabien's testing shows that that's
likely to hurt in a number of cases. I've some data suggesting the
contrary in others.

Things that'd be good:
* Some benchmarking. Right now controlled flushing is enabled by default on linux, but disabled by default on other
operatingsystems. Somebody running benchmarks on e.g. freebsd or OSX might be good.
 
* If somebody has the energy to provide a windows implemenation for flush control, that might be worthwhile. There's
severalplaces that could benefit from that.
 
* The default values are basically based on benchmarking by me and Fabien.

Regards,

Andres