Server-side copy with sendfile system call

[David Disseldorp]

On Fri, 23 May 2014 18:01:03 -0700, Richard Sharpe wrote:

> > Actually looking again at the graph, the drop in IOPS is suspiciously
> > consistent, occurring every 10-12 seconds. There's a good chance that
> > it coincides with a flush of the page-cache out to disk, in which case
> > you could try playing with your pdflush or IO-scheduler settings.
> > Also, Jeremy was looking at converting the code-path to use asynchronous
> > IO at one point. This would likely offer a significant increase in
> > performance for this workload, as it'd allow both client and server to
> > keep the IO pipeline full.  
> 
> I'm not sure I understand your reasoning for this last claim.
> 
> Surely, even with AIO, once the page cache is full, you will stall
> until some pages become free?

True, "significant increase" was probably a bit of an overstatement,
but with async copy-chunk handling and multi-credit, the client should
be able to queue multiple requests at the server while it's busy
flushing the page cache to disk.

Cheers, David