Re: [Qemu-devel] [PATCH 2/2] qapi: add block latency histogram interface

[Vladimir Sementsov-Ogievskiy]

06.02.2018 22:02, Eric Blake wrote:
> On 02/06/2018 12:06 PM, Vladimir Sementsov-Ogievskiy wrote:
> > 06.02.2018 18:50, Eric Blake wrote:
> > > On 02/06/2018 08:07 AM, Vladimir Sementsov-Ogievskiy wrote:
> > > > Set (and clear) histogram through new command
> > > > block-latency-histogram-set and show new statistics in
> > > > query-blockstats results.
> > >
> > > The commit message mentioned that you can set and clear histogram 
> > > tracking; how does this interface let you clear things?  By passing 
> > > a 0-length latency array?  If you violate the constraint (pass 
> > > non-ascending points, for example), does the previous latency map 
> > > get wiped out or is it still preserved unchanged?
> >
> > On error nothing is changed.
> >
> > By "clear" I mean zeroing statistics, not removing the whole 
> > histogram. And to zero statistics you can call set with the same 
> > latency array.
> > There is no way to remove histogram at all.. We can add 
> > block-latency-histogram-unset later if needed.
>
> Maybe "set (or restart) histogram collection points" might read 
> better? I also don't think we need a new command; if you make 
> 'latency' optional, then omitting it can serve to stop collecting 
> statistics altogether, without needing a new command (then again, if 
> you do that, now the command is used to "set, restart, and clear 
> histogram collection").

I like the idea, will do.

--
Best regards,
Vladimir