Re: Count backend self-sync calls

Jeff Janes wrote:
> Do you know where this competition is happening?  Is it on the
> platters, or is it in the hard drive write cache (I thought high-end
> hardware had tagged writes to avoid that), or in the kernel?
>   

Kernel.  Linux systems with lots of memory will happily queue up 
gigabytes of memory in their write cache, only getting serious about 
writing it out to disk when demanded to by fsync.


> This makes sense if we just need to append to a queue.  But once the
> queue is full and we are about to do a backend fsync, might it make
> sense to do a little more work to look for dups?
>   

One of the paths I'd like to follow is experimenting with both sorting 
writes by file and looking for duplication in the queues.  I think a 
basic, simple sync spreading approach needs to get finished first 
through; this sort of thing would then be an optimization on top of it.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us