Re: [Qemu-devel] [PATCH V2 0/4] *virtio-blk: add multiread support

[Peter Lieven]

On 16.12.2014 16:48, Kevin Wolf wrote:
> Am 16.12.2014 um 16:21 hat Peter Lieven geschrieben:
> > this series adds the long missing multiread support to virtio-blk.
> >
> > some remarks:
> >   - i introduced rd_merged and wr_merged block accounting stats to
> >     blockstats as a generic interface which can be set from any
> >     driver that will introduce multirequest merging in the future.
> >   - the knob to disable request merging is not yet there. I would
> >     add it to the device properties also as a generic interface
> >     to have the same switch for any driver that might introduce
> >     request merging in the future. As there has been no knob in
> >     the past I would post this as a seperate series as it needs
> >     some mangling in parameter parsing which might lead to further
> >     discussions.
> >   - the old multiwrite interface is still there and might be removed.
> >
> > v1->v2:
> >   - add overflow checking for nb_sectors [Kevin]
> >   - do not change the name of the macro of max mergable requests. [Fam]
> Diff to v1 looks good. Now I just need to check what it does to the
> performance. Did you run any benchmarks yourself?

I ran several installs of Debian/Ubuntu, Booting of Windows and Linux
systems. I looked at rd_total_time_ns and wr_total_time_ns and saw
no increase. Ofter I even saw even a decrease.

{rd,wr}_total_time_ns measures the time from virtio_blk_handle_request
to virtio_blk_rw_complete. So it seems to be a good indicator for the time
spent with I/O.

What you could to is post it on the top of your fio testing stack and
look at the throughput. Sequential Reads should be faster. The rest
not worse.

Peter