[libvirt] [PATCH v2] lib: Add note that bulk stats API queries may overrun RPC buffers

[Michal Privoznik]

On 05/24/2017 02:47 PM, Richard W.M. Jones wrote:
> 
> On Wed, May 24, 2017 at 12:49:58PM +0200, Michal Privoznik wrote:
>> That's quite exact. I mean the word 'guessing'. We can't really provide
>> reliable way of dealing with what you're suggesting (unless we cut the
>> limit really small) nor we can guarantee atomicity. Therefore I think it
>> would be a waste of time to work on this. Yes, it can be done, but the
>> benefits are pretty small IMO.
> 
> Why is atomicity a problem?

The atomicity steps in depending on what level we are talking about
serialization. If we do it the way Martin suggested (= on public API
level) then the list of domains may change between two iterations of the
suggested loop. Thus the result obtained would not reflect the reality.
However, if we are talking on RPC level, then there's no atomicity
problem as RPC is 'blind' to the data (it doesn't care what data is sent).

> Just structure the libvirtd
> messages so that you have:
> 
>   COLLECT_THE_STATS
>     - saves the stats into an internal buffer in libvirtd
>       and returns a handle and a number of stat items
>   RETURN_THE_STATS
>     - returns partial subset of previously collected stats,
>       called multiple times to transfer the data back to libvirt
>   FREE_THE_STATS
>     - free the internal buffer

This is exactly what I was proposing in my e-mail that I linked in the
other thread. You just wrote it more cute. Yet again, what's the
gain/advantage of this over one big message?

Michal