Re: poor domU VBD performance.

[Jens Axboe <axboe@suse.de>]

On Thu, Mar 31 2005, Kurt Garloff wrote:
> Hi,
> 
> On Thu, Mar 31, 2005 at 09:33:12AM -0500, Philip R Auld wrote:
> > This effects merging though, right? I don't think the the front
> > end has done any merging. 
> 
> The noop elevator does front and back merging.
> My understanding is that it's used in the frontend driver.
> 
> Otherwise, unplugging on every block would indeed be quite bad ...

Not necessarily - either your io rate is not fast enough to sustain a
substantial queue depth, in that case you get plugging on basically
every io anyways. If on the other hand the io rate is high enough to
maintain a queue depth of > 1, then the plugging will never take place
because the queue never empties.

So all in all, I don't think the temporary work-around will be such a
bad idea. I would still rather implement the queue tracking though, it
should not be more than a few lines of code.

And Philip, I will get the bio_sync() change merged :-)

-- 
Jens Axboe