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From: | Manos Pitsidianakis |
Subject: | Re: [Qemu-devel] [PATCH v3 4/7] block: remove legacy I/O throttling |
Date: | Fri, 8 Sep 2017 18:44:59 +0300 |
User-agent: | NeoMutt/20170609-57-1e93be (1.8.3) |
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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