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Re: [Qemu-devel] [PATCH] blk: fix aio context loss on media change

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] blk: fix aio context loss on media change
Date: Wed, 15 Mar 2017 16:30:01 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 15.03.2017 um 16:09 hat Paolo Bonzini geschrieben:
> >>>> There should be a policy on which BB sets AioContext on the BDS (e.g.
> >>>> only the device does it), but apart from that, it should not be an issue.
> >>>
> >>> We don't know which BBs are going to be attached. We don't necessarily
> >>> have a device at all, or we could have two of them.
> >>
> >> Wow, can we really have two? :-O
> > 
> > What would prevent you from doing this? The whole blockdev work was
> > about making the block layer more flexible, so now we have this
> > flexibility of attaching more or less anything to anything (unless op
> > blockers prevent it, which is why they are important for actually
> > supporting blockdev).
> Yeah, the actual question was more "will the blockers allow two" devices
> behind the same BDS.  But I suppose there's no reason to prevent that
> (emulating multipath, for example).

For read-only devices, there's no reason anyway. For writable ones, you
have to set the share-rw=on qdev property, but then it's allowed.

> >>> Though maybe we should try to keep a BDS and its children in the same
> >>> AioContext anyway if that's possible? Will it make a difference?
> >>
> >> Everything can make sense---but yes, keeping the whole hierarchy in the
> >> same AioContext makes sense more often.
> > 
> > So I take this to mean that it does make a difference. :-)
> > 
> > If we want to keep users and their child nodes in the same AioContext by
> > default, we'll probably still need to implement all of the callbacks
> > that we would need for proper AioContext management today.
> Or just assume that in the common case people won't specify iothread on
> -blockdev, only on -device.

Right, but then attaching a new device could mean that the BDS moves to
a different AioContext even though the other users are still on the old
one. Not sure if this is bad, but it might be unexpected.


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