From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52359) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WM3FZ-0003WS-BQ for qemu-devel@nongnu.org; Fri, 07 Mar 2014 17:28:21 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WM3FU-0000wW-Re for qemu-devel@nongnu.org; Fri, 07 Mar 2014 17:28:17 -0500 Received: from lnantes-156-75-100-125.w80-12.abo.wanadoo.fr ([80.12.84.125]:55922 helo=paradis.irqsave.net) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WM3FU-0000wR-HX for qemu-devel@nongnu.org; Fri, 07 Mar 2014 17:28:12 -0500 Date: Fri, 7 Mar 2014 23:28:10 +0100 From: =?iso-8859-1?Q?Beno=EEt?= Canet Message-ID: <20140307222810.GA3305@irqsave.net> References: <1394032700-1642-1-git-send-email-benoit.canet@irqsave.net> <1394032700-1642-2-git-send-email-benoit.canet@irqsave.net> <53178F14.80402@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <53178F14.80402@redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/2] block: Add node-name and to-replace-node-name arguments to drive-mirror. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eric Blake Cc: =?iso-8859-1?Q?Beno=EEt?= Canet , kwolf@redhat.com, Fam Zheng , qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, pbonzini@redhat.com The Wednesday 05 Mar 2014 =E0 13:54:44 (-0700), Eric Blake wrote : > On 03/05/2014 08:18 AM, Beno=EEt Canet wrote: > > node-name give a name to the created BDS and register it in the node = graph. >=20 > s/give/gives/ s/register/registers/ >=20 > >=20 > > to-replace-node-name can be used when drive-mirror is called with syn= c=3Dfull. > >=20 > > The purpose of these fields is to be able to reconstruct and replace = a broken > > quorum file. >=20 > There may be other uses possible from this, but the idea makes sense. >=20 > >=20 > > drive-mirror will bdrv_swap the new BDS named node-name with the one > > pointed by to-replace-node-name when the mirroring is finished. > >=20 > > Signed-off-by: Benoit Canet > > --- >=20 > > @@ -312,6 +313,10 @@ static void coroutine_fn mirror_run(void *opaque= ) > > s->common.len =3D bdrv_getlength(bs); > > if (s->common.len <=3D 0) { > > block_job_completed(&s->common, s->common.len); > > + /* Fam's new blocker API should be used here. */ > > + if (s->to_replace) { >=20 > Who is getting merged first? It seems like this should be fixed before > taking this patch, if Fam's work is indeed closer to inclusion. At any > rate, the comment seems odd - a year from now, Fam's work won't be new. >=20 > > + BlockDriverState *to_replace; > > + /* if a to-replace-node-name was specified use it's bs */ >=20 > s/it's/its/ - the rule is anywhere that you see "it's", re-read the > sentence with "it is" and see if it still makes sense; if not, you mean= t > "its". >=20 >=20 > > =20 > > static void mirror_start_job(BlockDriverState *bs, BlockDriverState = *target, > > + BlockDriverState *to_replace, > > int64_t speed, int64_t granularity, >=20 > Pre-existing, but as long as you are touching this, you might as well > fix indentation of the other lines in the same signature. >=20 > > @@ -2158,19 +2195,33 @@ void qmp_drive_mirror(const char *device, con= st char *target, > > return; > > } > > =20 > > + /* if we are planning to replace a graph node name the code shou= ld do a full > > + * mirror of the source image > > + */ > > + if (has_to_replace_node_name && sync !=3D MIRROR_SYNC_MODE_FULL)= { > > + error_setg(errp, > > + "to-replace-node-name can only be used with sync=3D= full"); > > + return; > > + } >=20 > I'm not sure I follow this restriction. What's to prevent me from doin= g > a shallow mirror coupled with the mode of reusing an existing file that > already points to a sane backing file, rather than forcing a full sync? > That is, why not let this command be a fully-generic swap command, > where the semantics are that as long as my old and new image have the > same contents from the guest's perspective (or I'm replacing a broken > file out of a quorum, and the new image has the same contents as the > quorum majority), then we are just updating qemu to point to a new BDS. >=20 > On the other hand, back around the 1.5 timeframe, downstream RHEL tried > to add a 'drive-reopen' command that did just that - replaced the > backing file of a guest's disk with an arbitrary other file. But it wa= s > so powerful and risky that at the time upstream finally added > 'transaction' support, we decided to go with the simpler > 'drive-mirror/block-job-complete' sequence as the only supported way to > cause qemu to associate a different BDS with a guest image. Of course, > things have advanced since then, so maybe we finally are at a point > where we want to expose a generic reopen command that can swap out > arbitrary named nodes without interrupting guest services, but now I'm > starting to wonder if it should be a new command instead of adding > optional arguments to the existing drive-mirror. Ok this feature won't go in QEMU 2.0. What about implementing as an additional drive-mirror-complete target-node-name=3Dno_name_to_replace new-node-name= =3Dnode_name_of_the_new_bs The command would do the same work as block-job-complete plus some extras work and would be used after starting a regular drive-mirror command. Best regards Beno=EEt >=20 > > +++ b/qapi-schema.json > > @@ -2140,6 +2140,14 @@ > > # @format: #optional the format of the new destination, default is t= o > > # probe if @mode is 'existing', else the format of the sour= ce > > # > > +# @new-node-name: #optional the new block driver state node name in = the graph > > +# (Since 2.1) >=20 > Ah, so you're not trying to get this in before 2.0 freeze - which means > we have more time to think about the implications. >=20 > > +# > > +# @to-replace-node-name: #optional with sync=3Dfull graph node name = to be > > +# replaced by the new image when a whole imag= e copy is > > +# done. This can be used to repair broken Quo= rum files. > > +# (Since 2.1) >=20 > This naming feels long, but I'm not sure if I have a better suggestion. > It looks like you only allow swapping out one quorum file per > drive-mirror - but what if I have a 3/5 quorum and want to swap out two > files at once? Also, how does this interact with the 'transaction' com= mand? >=20 > > ## > > { 'command': 'drive-mirror', > > 'data': { 'device': 'str', 'target': 'str', '*format': 'str', > > - 'sync': 'MirrorSyncMode', '*mode': 'NewImageMode', > > - '*speed': 'int', '*granularity': 'uint32', > > - '*buf-size': 'int', '*on-source-error': 'BlockdevOnError= ', > > + '*new-node-name': 'str', '*to-replace-node-name': 'str', > > + 'sync': 'MirrorSyncMode', '*mode': 'NewImageMode', '*spe= ed': 'int', > > + '*granularity': 'uint32', '*buf-size': 'int', > > + '*on-source-error': 'BlockdevOnError', >=20 > Why the reindent of existing options? >=20 > --=20 > Eric Blake eblake redhat com +1-919-301-3266 > Libvirt virtualization library http://libvirt.org >=20