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

[Manos Pitsidianakis <el13635@mail.ntua.gr>]

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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. Quoting a reply I 
made to Kevin on the interface test patch:

>> You're right, I missed this. The test result shows that this command
>> succeeds. Do we really want to allow other nodes to be affected with a
>> blockdev-add? Wouldn't it be cleaner to just forbid the combination of
>> limits and throtte-group?
>
>So basically only anonymous, immutable groups can be created through the 
>driver then. All other shared group configurations must be explicitly 
>created with an -object / object-add syntax. I think this is a neat 
>separation and compromise if we allow anonymous groups. If not, we can 
>ignore limits on the throttle driver.

So basically if we have anonymous groups, we accept limits in the driver 
options but only without a group-name. Without anonymous groups, we 
remove limits from the driver options and only use the 
object-add/-object commands to create throttle groups. Does this sound 
like a good idea? It will be more verbose for the human user. One 
advantage: all throttle groups can then be managed through 
qom-set/qom-get since they are owned by the qom tree.

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