From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60255) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1coAsN-0000Il-Lt for qemu-devel@nongnu.org; Wed, 15 Mar 2017 11:30:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1coAsM-0007an-OK for qemu-devel@nongnu.org; Wed, 15 Mar 2017 11:30:11 -0400 Date: Wed, 15 Mar 2017 16:30:01 +0100 From: Kevin Wolf Message-ID: <20170315153001.GM4030@noname.str.redhat.com> References: <20170314171120.80741-1-vsementsov@virtuozzo.com> <20170315110351.GG4030@noname.str.redhat.com> <9a68a30f-8765-ca91-572f-ed8d7083b202@redhat.com> <20170315143032.GK4030@noname.str.redhat.com> <87bb192d-b910-5ac2-886b-fefeb4a2ec4d@redhat.com> <20170315150210.GL4030@noname.str.redhat.com> <1f49061f-a73c-35ea-9d8b-89d21e22cbfe@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1f49061f-a73c-35ea-9d8b-89d21e22cbfe@redhat.com> Subject: Re: [Qemu-devel] [PATCH] blk: fix aio context loss on media change List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org, mreitz@redhat.com, jsnow@redhat.com, famz@redhat.com, den@openvz.org, stefanha@redhat.com 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. Kevin