Re: [Qemu-devel] [Qemu-block] [PATCH] qed: fix bdrv_qed_drain

[Stefan Hajnoczi]

On Mon, Mar 07, 2016 at 10:22:58PM +0100, Paolo Bonzini wrote:
> On 07/03/2016 21:56, Stefan Hajnoczi wrote:
> > I think the timer concept itself is troublesome.  A radical approach but
> > limited to changing QED itself is to drop the timer and instead keep a
> > timestamp for the last allocating write request.  Next time a write
> > request (allocating or rewriting) is about to complete, do the flush and
> > clear the need check flag as part of the write request (if 5 seconds
> > have passed since the timestamp).
> 
> bdrv_qed_drain should be easy to fix in my new drain implementation,
> which is based on draining the parent before the BdrvChild-ren.  It's
> just troublesome in the current one which alternates flushing and draining.
> 
> I would just revert the patch that introduced bdrv_qed_drain for now,
> and reintroduce it later (note however that something was messed up in
> commit df9a681, "qed: Implement .bdrv_drain", 2015-11-12, because it
> includes some dirty bitmap stuff).

You're right, that might be the best solution for the time being.  AFAIK
the need check write is harmless.  It does not result in a guest-visible
change and is basically just a flush + header update.

Stefan

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