Re: [Qemu-devel] [PATCH 2/4] block: immediately cancel oversized read/write requests

[Kevin Wolf]

Am 23.09.2014 um 10:55 hat Peter Lieven geschrieben:
> >>  - should we only merge requests within the same cluster?
> >Does it hurt to merge everything we can? The block driver needs to be
> >able to take things apart anyway, the large request could come from
> >somewhere else (guest, block job, builtin NBD server, etc.)
> 
> I was just thinking. It was unclear what the original intention was.

I hope it's clearer now. Anyway, for qcow2 merging even across clusters
does help a bit if those clusters are physically contiguous.

> My concern was that merging too much will increase latency for
> the specific I/O even if it increases throughput.

That's probably a valid concern. Perhaps we should add a set of options
to enable or disable certain request manipulations that the block layer
can do.  This includes request merging, but also alignment adjustments,
zero detection (which already has an option) etc.

> >>  - why is there no multiread_merge?
> >Because nobody implemented it. :-)
> 
> Maybe its worth looking at this. Taking the multiwrite_merge stuff as
> a basis it shouldn't be too hard.

Yes, it should be doable. We need numbers, though, before merging
something like this.

> >As I said above, writes hurt a lot because of qcow2 cluster allocation.
> >Reads are probably losing some performance as well (someone would need
> >to check this), but processing them separately isn't quite as painful.
> 
> I will try to sort that out.

Great, thanks!

Kevin