Re: [Qemu-block] [PATCH v3 5/7] block: add throttle block filter driver

[Alberto Garcia]

On Tue 08 Aug 2017 03:45:44 PM CEST, Manos Pitsidianakis wrote:
> On Tue, Aug 08, 2017 at 03:13:36PM +0200, Alberto Garcia wrote:
>>On Mon 31 Jul 2017 11:54:41 AM CEST, Manos Pitsidianakis wrote:
>>> block/throttle.c uses existing I/O throttle infrastructure inside a
>>> block filter driver. I/O operations are intercepted in the filter's
>>> read/write coroutines, and referred to block/throttle-groups.c
>>>
>>> The driver can be used with the syntax
>>> -drive driver=throttle,file.filename=foo.qcow2, \
>>>         limits.iops-total=...,throttle-group=bar
>>
>>Sorry for not having noticed this earlier, but can't you define the
>>throttling group (and its limits) using -object throttle-group ... as
>>shown in the previous patch, and simply reference it here? Or would we
>>have two alternative ways of setting the throttling limits?
>>
>>What happens if you have many -drive lines each one with a different set
>>of limits but with the same throttling group?
>
> The limits of the last one to be processed will win.

That's what the current throttling API does, and I tend to agree that
it's not completely straightforward (a few people have asked me the same
question since this feature is available).

If we're going to add a new API we could eliminate this ambiguity. After
all the previous -drive throttling.iops-total=... would still be
available, wouldn't it?

> So basically if we have anonymous groups, we accept limits in the
> driver options but only without a group-name.

In the commit message you do however have limits and a group name, is
that a mistake?

    -drive driver=throttle,file.filename=foo.qcow2, \
           limits.iops-total=...,throttle-group=bar

Berto