[libvirt] [RFC PATCHv2 1/8] threshold: new API virDomainBlockSetWriteThreshold

[Francesco Romani]

----- Original Message -----
> From: "Eric Blake" <eblake at redhat.com>
> To: "Peter Krempa" <pkrempa at redhat.com>
> Cc: libvir-list at redhat.com, fromani at redhat.com
> Sent: Monday, June 15, 2015 6:21:13 PM
> Subject: Re: [libvirt] [RFC PATCHv2 1/8] threshold: new API	virDomainBlockSetWriteThreshold
> 
> On 06/15/2015 07:19 AM, Peter Krempa wrote:
> > On Fri, Jun 12, 2015 at 13:29:25 -0600, Eric Blake wrote:
> >> qemu 2.3 added a new QMP command block-set-write-threshold,
> >> which allows callers to get an interrupt when a file hits a
> >> write threshold, rather than the current approach of repeatedly
> >> polling for file allocation.  This patch prepares the API for
> >> callers to register to receive the event, as well as a way
> >> to query the threshold via virDomainListGetStats().
> >>
> 
> >> +
> >> +typedef enum {
> >> +    /* threshold is thousandth of a percentage (0 to 100000) relative to
> > 
> > You managed to choose a unusual unit. Commonly used ones are 1/1000 and
> > 1/1 000 000. Financial world also uses 1/10 000. Your unit of 1/100 000
> > is not among:
> > 
> > https://en.wikipedia.org/wiki/Parts-per_notation#Parts-per_expressions
> > 
> > I'd again suggest to use 1/1 000 000. Or if you want to be uber preciese
> > you might choose 1/(2^64 - 1).
> 
> Francesco, what precision would you like?  Parts per million seems okay
> to me, if we want an order of magnitude closer; and I don't think we
> need anything beyond that.  Or if parts per thousand is sufficient, that
> leads to smaller numbers on input.  But it's pretty trivial for me to
> adjust the code to a different base, for whatever people would like.

We (in oVirt) use very coarse thresholds.
For our current needs, I believe even parts per thousand is sufficient.
Trying to be a bit forward thinking, I believe parts per million is perfectly fine.

Bests,

-- 
Francesco Romani
RedHat Engineering Virtualization R & D
Phone: 8261328
IRC: fromani