Re: [Qemu-devel] [PATCH 2/2 v2] Direct IDE I/O

[Paul Brook <paul@codesourcery.com>]

On Monday 03 December 2007, Markus Hitter wrote:
> Am 03.12.2007 um 11:30 schrieb Laurent Vivier:
> > But if you think I should remove the buffered case, I can.
>
> In doubt, less code is always better. For the unlikely case you broke
> something badly, there's always the option to take back the patch.
>
> > BTW, do you think I should enable "cache=off" by default ?
>
> This would be fine for a transition phase, but likely, the cache=on
> case gets forgotten to be removed later. So, do it now.

I think host caching is still useful enough to be enabled by default, and 
provides a significant performance increase in several cases. 

- The guest typically has a relatively small quantity of RAM, compared to a 
modern machine.  Allowing the host OS to act as a demand-based L2 cache 
allows this to be used without having to dedicate excessive quantities of ram 
to qemu.
- I've seen reports that it significantly speeds up the windows installer.
- Host cache is persistent between multiple qemu runs. f you're doing anything 
that requires frequent guest reboots (e.g. kernel debugging) this is going to 
be a huge win.
- You're running a host OS that has limited or no caching (e.g. DOS).

I'd hope that the host OS would have cache use heuristics that would help 
limit cache pollution.

Paul