Re: Testing Sandforce SSD

Greg Spiegelberg wrote:
> I know I'm talking development now but is there a case for a pg_xlog
> block device to remove the file system overhead and guaranteeing your
> data is written sequentially every time?

It's possible to set the PostgreSQL wal_sync_method parameter in the
database to open_datasync or open_sync, and if you have an operating
system that supports direct writes it will use those and bypass things
like the OS write cache. That's close to what you're suggesting,
supposedly portable, and it does show some significant benefit when it's
properly supported. Problem has been, the synchronous writing code on
Linux in particular hasn't ever worked right against ext3, and the
PostgreSQL code doesn't make the right call at all on Solaris. So
there's two popular platforms that it just plain doesn't work on, even
though it should.

We've gotten reports that there are bleeding edge Linux kernel and
library versions available now that finally fix that issue, and that
PostgreSQL automatically takes advantage of them when it's compiled on
one of them. But I'm not aware of any distribution that makes this easy
to try out that's available yet, paint is still wet on the code I think.

--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us