From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50931) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z7lLf-0005FA-6n for qemu-devel@nongnu.org; Wed, 24 Jun 2015 10:08:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z7lLZ-0004wB-CK for qemu-devel@nongnu.org; Wed, 24 Jun 2015 10:08:19 -0400 Date: Wed, 24 Jun 2015 15:07:56 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20150624140755.GC2206@work-vm> References: <1434617361-17778-1-git-send-email-wency@cn.fujitsu.com> <1434617361-17778-8-git-send-email-wency@cn.fujitsu.com> <20150618125533.GE25387@stefanha-thinkpad.redhat.com> <5582D777.8060202@gmail.com> <20150618160618.GF822@stefanha-thinkpad.redhat.com> <55836860.2010401@cn.fujitsu.com> <20150619104953.GB9807@stefanha-thinkpad.redhat.com> <5584DEA8.80005@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5584DEA8.80005@gmail.com> Subject: Re: [Qemu-devel] [Qemu-block] [PATCH COLO-Block v6 07/16] Add new block driver interface to connect/disconnect the remote target List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wen Congyang Cc: Kevin Wolf , Yang Hongyang , Fam Zheng , Lai Jiangshan , Stefan Hajnoczi , Jiang Yunhong , Dong Eddie , qemu devel , Max Reitz , Gonglei , Stefan Hajnoczi , Paolo Bonzini , qemu block , zhanghailiang * Wen Congyang (ghostwcy@gmail.com) wrote: > At 2015/6/19 18:49, Stefan Hajnoczi Wrote: > >On Fri, Jun 19, 2015 at 08:54:56AM +0800, Wen Congyang wrote: > >>On 06/19/2015 12:06 AM, Stefan Hajnoczi wrote: > >>>On Thu, Jun 18, 2015 at 10:36:39PM +0800, Wen Congyang wrote: > >>>>At 2015/6/18 20:55, Stefan Hajnoczi Wrote: > >>>>>On Thu, Jun 18, 2015 at 04:49:12PM +0800, Wen Congyang wrote: > >>>>>>+void bdrv_connect(BlockDriverState *bs, Error **errp) > >>>>>>+{ > >>>>>>+ BlockDriver *drv = bs->drv; > >>>>>>+ > >>>>>>+ if (drv && drv->bdrv_connect) { > >>>>>>+ drv->bdrv_connect(bs, errp); > >>>>>>+ } else if (bs->file) { > >>>>>>+ bdrv_connect(bs->file, errp); > >>>>>>+ } else { > >>>>>>+ error_setg(errp, "this feature or command is not currently supported"); > >>>>>>+ } > >>>>>>+} > >>>>>>+ > >>>>>>+void bdrv_disconnect(BlockDriverState *bs) > >>>>>>+{ > >>>>>>+ BlockDriver *drv = bs->drv; > >>>>>>+ > >>>>>>+ if (drv && drv->bdrv_disconnect) { > >>>>>>+ drv->bdrv_disconnect(bs); > >>>>>>+ } else if (bs->file) { > >>>>>>+ bdrv_disconnect(bs->file); > >>>>>>+ } > >>>>>>+} > >>>>> > >>>>>Please add doc comments describing the semantics of these commands. > >>>> > >>>>Where should it be documented? In the header file? > >>> > >>>block.h doesn't document prototypes in the header file, please document > >>>the function definition in block.c. (QEMU is not consistent here, some > >>>places do it the other way around.) > >>> > >>>>>Why are these operations needed when there is already a bs->drv == NULL > >>>>>case which means the BDS is not ready for read/write? > >>>>> > >>>> > >>>>The purpos is that: don't connect to nbd server when opening a nbd client. > >>>>connect/disconnect > >>>>to nbd server when we need to do it. > >>>> > >>>>IIUC, if bs->drv is NULL, it means that the driver is ejected? Here, > >>>>connect/disconnect > >>>>means that connect/disconnect to remote target(The target may be in another > >>>>host). > >>> > >>>Connect/disconnect puts something on the QEMU command-line that isn't > >>>ready at startup time. > >>> > >>>How about using monitor commands to add objects when needed instead? > >>> > >>>That is cleaner because it doesn't introduce a new state (which is only > >>>implemented for nbd). > >>> > >> > >>The problem is that, nbd client is one child of quorum, and quorum must have more > >>than one child. The nbd server is not ready until colo is running. > > > >A monitor command to hot add/remove quorum children solves this problem > >and could also be used in other scenarios (e.g. user decides to take a > >quorum child offline). > > > > For replication case, we always do checkpoint again and again after > migration. If the disk is not synced before migration, we will use disk mirgation or mirror > job to sync it. Can you document the way that you use disk migration or mirror, together with your COLO setup? I think it would make it easier to understand this restriction. At the moment I don't understand how you can switch from doing a disk migration into COLO mode - there seems to be a gap between the end of disk migration and the start of COLO. > So we cannot start block replication when migration is running. We need > that nbd > client is not ready when migration is running, and it is ready between > migration ends > and checkpoint begins. Using a monotir command add the nbd client will cause > larger > downtime. So if the nbd client has been added(only not connect to the nbd > server), > we can connect to nbd server automatically. Without the disk migration or mirror, I can't see the need for the delayed bdrv_connect. I can see that you want to do a disconnect at failover, however you need to take care because if the network is broken at the point of failover you need to make sure nothing blocks. Dave > > Thanks > Wen Congyang -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK