Re: [Qemu-block] [PATCH v3 4/7] block: remove legacy I/O throttling

[Manos Pitsidianakis]

On Thu, Sep 07, 2017 at 03:26:11PM +0200, Kevin Wolf wrote:
> We shouldn't really need any throttling code in
> blk_root_drained_begin/end any more now because the throttle node will
> be drained. If this code is necessary, a bdrv_drain() on an explicit
> throttle node will work differently from one on an implicit one.
>
> Unfortunately, this seems to be true about the throttle node. Implicit
> throttle nodes will keep ignoring the throttle limit in order to
> complete the drain request quickly, where as explicit throttle nodes
> will process their requests at the configured speed before the drain
> request can be completed.
>
> This doesn't feel right to me, both should behave the same.
>
> Kevin

I suppose we can implement bdrv_co_drain and increase io_limits_disabled 
from inside the driver. And then remove the implicit filter logic from 
blk_root_drained_begin. But there's no _end callback equivalent so we 
can't decrease io_limits_disabled at the end of the drain. So I think 
there are two options:

- make a bdrv_co_drain_end cb and recurse in blk_root_drain_end for all 
 children to call it. Old behavior of I/O bursts (?) during a drain is 
 kept.
- remove io_limits_disabled and let throttled requests obey limits 
 during a drain

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