RE: [PATCH v6 4/4] colo: Update Documentation for continuous replication

[Zhang, Chen]

> -----Original Message-----
> From: Lukas Straub <address@hidden>
> Sent: Saturday, October 12, 2019 12:01 AM
> To: Zhang, Chen <address@hidden>
> Cc: qemu-devel <address@hidden>; Jason Wang
> <address@hidden>; Wen Congyang <address@hidden>;
> Xie Changlong <address@hidden>; Kevin Wolf
> <address@hidden>; Max Reitz <address@hidden>; qemu-block
> <address@hidden>
> Subject: Re: [PATCH v6 4/4] colo: Update Documentation for continuous
> replication
> 
> On Thu, 10 Oct 2019 10:34:15 +0000
> "Zhang, Chen" <address@hidden> wrote:
> 
> > > -----Original Message-----
> > > From: Lukas Straub <address@hidden>
> > > Sent: Wednesday, October 9, 2019 11:17 PM
> > > To: Zhang, Chen <address@hidden>
> > > Cc: qemu-devel <address@hidden>; Jason Wang
> > > <address@hidden>; Wen Congyang
> <address@hidden>; Xie
> > > Changlong <address@hidden>; Kevin Wolf
> <address@hidden>;
> > > Max Reitz <address@hidden>; qemu-block <qemu-
> address@hidden>
> > > Subject: Re: [PATCH v6 4/4] colo: Update Documentation for
> > > continuous replication
> > >
> > > On Wed, 9 Oct 2019 08:36:52 +0000
> > > "Zhang, Chen" <address@hidden> wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Lukas Straub <address@hidden>
> > > > > Sent: Saturday, October 5, 2019 9:06 PM
> > > > > To: qemu-devel <address@hidden>
> > > > > Cc: Zhang, Chen <address@hidden>; Jason Wang
> > > > > <address@hidden>; Wen Congyang
> > > <address@hidden>; Xie
> > > > > Changlong <address@hidden>; Kevin Wolf
> > > <address@hidden>;
> > > > > Max Reitz <address@hidden>; qemu-block <qemu-
> > > address@hidden>
> > > > > Subject: [PATCH v6 4/4] colo: Update Documentation for
> > > > > continuous replication
> > > > >
> > > > > Document the qemu command-line and qmp commands for
> continuous
> > > > > replication
> > > > >
> > > > > Signed-off-by: Lukas Straub <address@hidden>
> > > > > ---
> > > > >  docs/COLO-FT.txt           | 213 +++++++++++++++++++++++++++------
> ----
> > > > >  docs/block-replication.txt |  28 +++--
> > > > >  2 files changed, 174 insertions(+), 67 deletions(-)
> > > > >
> > > > > diff --git a/docs/COLO-FT.txt b/docs/COLO-FT.txt index
> > > > > ad24680d13..bc1a0ccb99 100644
> > > > > --- a/docs/COLO-FT.txt
> > > > > +++ b/docs/COLO-FT.txt
> > > > > @@ -145,35 +145,65 @@ The diagram just shows the main qmp
> > > command,
> > > > > you can get the detail  in test procedure.
> > > > >
> > > > > ...
> > > > >
> > > > > +Note: Here we are running both instances on the same Host for
> > > > > +testing, change the IP Addresses if you want to run it on two
> > > > > +Hosts. Initally
> > > > > +127.0.0.1 is the Primary Host and 127.0.0.2 is the Secondary Host.
> > > > > +
> > > > > +== Startup qemu ==
> > > > > +1. Primary:
> > > > > +Note: Initally, $imagefolder/primary.qcow2 needs to be copied
> > > > > +to all
> > > Hosts.
> > > > > +# imagefolder="/mnt/vms/colo-test-primary"
> > > > > +
> > > > > +# qemu-system-x86_64 -enable-kvm -cpu qemu64,+kvmclock -m
> 512 -
> > > smp
> > > > > 1 -qmp stdio \
> > > > > +   -device piix3-usb-uhci -device usb-tablet -name primary \
> > > > > +   -netdev
> > > > > + tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper
> > > > > \
> > > > > +   -device rtl8139,id=e0,netdev=hn0 \
> > > > > +   -chardev socket,id=mirror0,host=0.0.0.0,port=9003,server,nowait \
> > > > > +   -chardev
> > > > > + socket,id=compare1,host=0.0.0.0,port=9004,server,wait \
> > > >
> > > > We should change the host=127.0.0.1 consistent with the expression
> below.
> > >
> > > Hi,
> > > This (and the IPs below in the QMP commands) needs to be this way,
> > > because it's a listening port and with 127.0.0.1 it would only
> > > listen on the loopback ip and wouldn't be reachable from another
> > > node for example. With
> > > 0.0.0.0 it will listen on all Interfaces.
> >
> > Yes, I know.  For this command demo, maybe use 192.168.0.1/192.168.0.2
> are more clear.
> 
> Hmm,
> the compare0 and compare_out actually can be replaced by unix sockets.
> So what do you think about the following?
> 
>    -chardev socket,id=mirror0,host=127.0.0.1,port=9003,server,nowait \
>    -chardev socket,id=compare1,host=127.0.0.1,port=9004,server,wait \
>    -chardev socket,id=compare0,path=/tmp/compare0.sock,server,nowait \
>    -chardev socket,id=compare0-0,path=/tmp/compare0.sock \
>    -chardev
> socket,id=compare_out,path=/tmp/compare_out.sock,server,nowait \
>    -chardev socket,id=compare_out0,path=/tmp/compare_out.sock \

In this way, user must create the unix socket node before start COLO, it is 
very hard to use.
I re-considered the issue here, looks keep the 0.0.0.0 is a relatively 
reasonably choice.
We can add some note here like you said before, to make sure user know the 
means of 0.0.0.0. 

Thanks
Zhang Chen

> 
> > >
> > > > > +   -chardev
> > > > > + socket,id=compare0,host=127.0.0.1,port=9001,server,nowait
> > > \
> > > > > +   -chardev socket,id=compare0-0,host=127.0.0.1,port=9001 \
> > > > > +   -chardev
> > > > > + socket,id=compare_out,host=127.0.0.1,port=9005,server,nowait
> > > > > \
> > > > > +   -chardev socket,id=compare_out0,host=127.0.0.1,port=9005 \
> > > > > +   -object filter-mirror,id=m0,netdev=hn0,queue=tx,outdev=mirror0
> \
> > > > > +   -object filter-
> > > > > redirector,netdev=hn0,id=redire0,queue=rx,indev=compare_out \
> > > > > +   -object filter-
> > > > > redirector,netdev=hn0,id=redire1,queue=rx,outdev=compare0 \
> > > > > +   -object iothread,id=iothread1 \
> > > > > +   -object
> > > > > +colo-compare,id=comp0,primary_in=compare0-
> > > > > 0,secondary_in=compare1,\
> > > > > +outdev=compare_out0,iothread=iothread1 \
> > > > > +   -drive
> > > > > +if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-thres
> > > > > +hold
> > > > > +=1,\
> > > > > +children.0.file.filename=$imagefolder/primary.qcow2,children.0.
> > > > > +driv
> > > > > +er=q
> > > > > +cow2 -S
> > > > > +
> > > > > +2. Secondary:
> > > > > +# imagefolder="/mnt/vms/colo-test-secondary"
> > > > > +# primary_ip=127.0.0.1
> > > > > +
> > > > > +# qemu-img create -f qcow2 $imagefolder/secondary-active.qcow2
> > > > > +10G
> > > > > +
> > > > > +# qemu-img create -f qcow2 $imagefolder/secondary-hidden.qcow2
> > > 10G
> > > > > +
> > > >
> > > > The active disk and hidden disk just need create one time, we can
> > > > note that
> > > here.
> > >
> > > Ok, I will Note that. But I will wait until the block changes are
> > > reviewed before sending the next version.
> >
> > That's fine for me.
> >
> > Thanks
> > Zhang Chen
> >
> > >
> > > Regards,
> > > Lukas Straub
> > >
> > > > > +# qemu-system-x86_64 -enable-kvm -cpu qemu64,+kvmclock -m
> 512 -
> > > smp
> > > > > 1 -qmp stdio \
> > > > > +   -device piix3-usb-uhci -device usb-tablet -name secondary \
> > > > > +   -netdev
> > > > > + tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper
> > > > > \
> > > > > +   -device rtl8139,id=e0,netdev=hn0 \
> > > > > +   -chardev socket,id=red0,host=$primary_ip,port=9003,reconnect=1
> \
> > > > > +   -chardev socket,id=red1,host=$primary_ip,port=9004,reconnect=1
> \
> > > > > +   -object filter-redirector,id=f1,netdev=hn0,queue=tx,indev=red0 \
> > > > > +   -object filter-redirector,id=f2,netdev=hn0,queue=rx,outdev=red1
> \
> > > > > +   -object filter-rewriter,id=rew0,netdev=hn0,queue=all \
> > > > > +   -drive
> > > > > if=none,id=parent0,file.filename=$imagefolder/primary.qcow2,driv
> > > > > er=q
> > > > > cow
> > > > > 2 \
> > > > > +   -drive
> > > > > +if=none,id=childs0,driver=replication,mode=secondary,file.drive
> > > > > +r=qc
> > > > > +ow2,
> > > > > +\
> > > > > +top-id=childs0,file.file.filename=$imagefolder/secondary-active
> > > > > +.qco
> > > > > +w2,\
> > > > > +file.backing.driver=qcow2,file.backing.file.filename=$imagefold
> > > > > +er/s
> > > > > +econ
> > > > > +dary-hidden.qcow2,\
> > > > > +file.backing.backing=parent0 \
> > > > > +   -drive
> > > > > +if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-thres
> > > > > +hold
> > > > > +=1,\
> > > > > +children.0=childs0 \
> > > > > +   -incoming tcp:0.0.0.0:9998
> > > > > +
> > > > > +
> > > > > +3. On Secondary VM's QEMU monitor, issue command
> > > > >  {'execute':'qmp_capabilities'}
> > > > > -{ 'execute': 'nbd-server-start',
> > > > > -  'arguments': {'addr': {'type': 'inet', 'data': {'host': 
> > > > > 'xx.xx.xx.xx', 'port':
> > > > > '8889'} } } -}
> > > > > -{'execute': 'nbd-server-add', 'arguments': {'device':
> > > > > 'secondary-disk0',
> > > > > 'writable': true } }
> > > > > +{'execute': 'nbd-server-start', 'arguments': {'addr': {'type':
> > > > > +'inet',
> > > > > +'data': {'host': '0.0.0.0', 'port': '9999'} } } }
> > > > > +{'execute': 'nbd-server-add', 'arguments': {'device':
> > > > > +'parent0',
> > > > > +'writable': true } }
> > > > >
> > > > >  Note:
> > > > >    a. The qmp command nbd-server-start and nbd-server-add must
> > > > > be run @@ -182,44 +212,113 @@ Note:
> > > > >       same.
> > > > >    c. It is better to put active disk and hidden disk in ramdisk.
> > > > >
> > > > > -3. On Primary VM's QEMU monitor, issue command:
> > > > > +4. On Primary VM's QEMU monitor, issue command:
> > > > >  {'execute':'qmp_capabilities'}
> > > > > -{ 'execute': 'human-monitor-command',
> > > > > -  'arguments': {'command-line': 'drive_add -n buddy
> > > > > driver=replication,mode=primary,file.driver=nbd,file.host=xx.xx.
> > > > > xx.x
> > > > > x,file.p
> > > > > ort=8889,file.export=secondary-disk0,node-name=nbd_client0'}}
> > > > > -{ 'execute':'x-blockdev-change', 'arguments':{'parent':
> > > > > 'primary-disk0',
> > > > > 'node': 'nbd_client0' } } -{ 'execute': 'migrate-set-capabilities',
> > > > > -      'arguments': {'capabilities': [ {'capability': 'x-colo', 
> > > > > 'state': true } ] } }
> > > > > -{ 'execute': 'migrate', 'arguments': {'uri': 'tcp:xx.xx.xx.xx:8888'
> > > > > } }
> > > > > +{'execute': 'human-monitor-command', 'arguments': {'command-
> line':
> > > > > +'drive_add -n buddy
> > > > > +driver=replication,mode=primary,file.driver=nbd,file.host=127.0
> > > > > +.0.2 ,fil
> > > > > +e.port=9999,file.export=parent0,node-name=replication0'}}
> > > > > +{'execute': 'x-blockdev-change', 'arguments':{'parent':
> > > > > +'colo-disk0',
> > > > > +'node': 'replication0' } }
> > > > > +{'execute': 'migrate-set-capabilities', 'arguments':
> > > > > +{'capabilities': [
> > > > > +{'capability': 'x-colo', 'state': true } ] } }
> > > > > +{'execute': 'migrate', 'arguments': {'uri':
> > > > > +'tcp:127.0.0.2:9998' } }
> > > > >
> > > > >    Note:
> > > > >    a. There should be only one NBD Client for each primary disk.
> > > > > -  b. xx.xx.xx.xx is the secondary physical machine's hostname
> > > > > or IP
> > > > > -  c. The qmp command line must be run after running qmp command
> > > > > line in
> > > > > +  b. The qmp command line must be run after running qmp
> command
> > > > > + line in
> > > > >       secondary qemu.
> > > > >
> > > > > -4. After the above steps, you will see, whenever you make
> > > > > changes to PVM, SVM will be synced.
> > > > > +5. After the above steps, you will see, whenever you make
> > > > > +changes to PVM,
> > > > > SVM will be synced.
> > > > >  You can issue command '{ "execute": "migrate-set-parameters" ,
> > > > > "arguments":{ "x-checkpoint-delay": 2000 } }'
> > > > > -to change the checkpoint period time
> > > > > +to change the idle checkpoint period time
> > > > > +
> > > > > +6. Failover test
> > > > > +You can kill one of the VMs and Failover on the surviving VM:
> > > > > +
> > > > > +If you killed the Secondary, then follow "Primary Failover".
> > > > > +After that, if you want to resume the replication, follow
> > > > > +"Primary resume
> > > > > replication"
> > > > > +
> > > > > +If you killed the Primary, then follow "Secondary Failover".
> > > > > +After that, if you want to resume the replication, follow
> > > > > +"Secondary resume
> > > > > replication"
> > > > > +
> > > > > +== Primary Failover ==
> > > > > +The Secondary died, resume on the Primary
> > > > > +
> > > > > +{'execute': 'x-blockdev-change', 'arguments':{ 'parent':
> > > > > +'colo-disk0',
> > > > > +'child': 'children.1'} }
> > > > > +{'execute': 'human-monitor-command', 'arguments':{ 'command-
> line':
> > > > > +'drive_del replication0' } }
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'comp0' } }
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'iothread1' } }
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'm0' } }
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'redire0' } }
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'redire1' } }
> > > > > +{'execute': 'x-colo-lost-heartbeat' }
> > > > > +
> > > > > +== Secondary Failover ==
> > > > > +The Primary died, resume on the Secondary and prepare to become
> > > > > +the
> > > > > new
> > > > > +Primary
> > > > > +
> > > > > +{'execute': 'nbd-server-stop'}
> > > > > +{'execute': 'x-colo-lost-heartbeat'}
> > > > > +
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'f2' } }
> > > > > +{'execute': 'object-del', 'arguments':{ 'id': 'f1' } }
> > > > > +{'execute': 'chardev-remove', 'arguments':{ 'id': 'red1' } }
> > > > > +{'execute': 'chardev-remove', 'arguments':{ 'id': 'red0' } }
> > > > > +
> > > > > +{'execute': 'chardev-add', 'arguments':{ 'id': 'mirror0', 'backend':
> > > > > +{'type': 'socket', 'data': {'addr': { 'type': 'inet', 'data': { 
> > > > > 'host':
> > > > > +'0.0.0.0', 'port': '9003' } }, 'server': true } } } }
> > > >
> > > > Same like I said before.
> > > >
> > > > Others statement looks good for me.
> > > >
> > > > Thanks
> > > > Zhang Chen
> > > >
> > > > > +{'execute': 'chardev-add', 'arguments':{ 'id': 'compare1', 'backend':
> > > > > +{'type': 'socket', 'data': {'addr': { 'type': 'inet', 'data': { 
> > > > > 'host':
> > > > > +'0.0.0.0', 'port': '9004' } }, 'server': true } } } }
> > > > > +{'execute': 'chardev-add', 'arguments':{ 'id': 'compare0', 'backend':
> > > > > +{'type': 'socket', 'data': {'addr': { 'type': 'inet', 'data': { 
> > > > > 'host':
> > > > > +'127.0.0.1', 'port': '9001' } }, 'server': true } } } }
> > > > > +{'execute': 'chardev-add', 'arguments':{ 'id': 'compare0-0', 
> > > > > 'backend':
> > > > > +{'type': 'socket', 'data': {'addr': { 'type': 'inet', 'data': { 
> > > > > 'host':
> > > > > +'127.0.0.1', 'port': '9001' } }, 'server': false } } } }
> > > > > +{'execute': 'chardev-add', 'arguments':{ 'id': 'compare_out',
> > > > > +'backend': {'type': 'socket', 'data': {'addr': { 'type':
> > > > > +'inet',
> > > > > +'data': { 'host': '127.0.0.1', 'port': '9005' } }, 'server':
> > > > > +true } } } }
> > > > > +{'execute': 'chardev-add', 'arguments':{ 'id': 'compare_out0',
> > > > > +'backend': {'type': 'socket', 'data': {'addr': { 'type':
> > > > > +'inet',
> > > > > +'data': { 'host': '127.0.0.1', 'port': '9005' } }, 'server':
> > > > > +false } } } }
> > > > > +
> > > > > +== Primary resume replication == Resume replication after new
> > > > > +Secondary is up.
> > > > > +
> > > > > +Start the new Secondary (Steps 2 and 3 above), then on the Primary:
> > > > > +{'execute': 'drive-mirror', 'arguments':{ 'device':
> > > > > +'colo-disk0',
> > > > > +'job-id': 'resync', 'target': 'nbd://127.0.0.2:9999/parent0', 'mode':
> > > > > +'existing', 'format': 'raw', 'sync': 'full'} }
> > > > > +
> > > > > +Wait until disk is synced, then:
> > > > > +{'execute': 'stop'}
> > > > > +{'execute': 'block-job-cancel', 'arguments':{ 'device':
> > > > > +'resync'} }
> > > > > +
> > > > > +{'execute': 'human-monitor-command', 'arguments':{ 'command-
> line':
> > > > > +'drive_add -n buddy
> > > > > +driver=replication,mode=primary,file.driver=nbd,file.host=127.0
> > > > > +.0.2 ,fil
> > > > > +e.port=9999,file.export=parent0,node-name=replication0'}}
> > > > > +{'execute': 'x-blockdev-change', 'arguments':{ 'parent':
> > > > > +'colo-disk0',
> > > > > +'node': 'replication0' } }
> > > > > +
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'filter-mirror',
> > > > > +'id': 'm0', 'props': { 'netdev': 'hn0', 'queue': 'tx', 'outdev':
> > > > > +'mirror0' } } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'filter-redirector', 'id': 'redire0', 'props': { 'netdev':
> > > > > +'hn0',
> > > > > +'queue': 'rx', 'indev': 'compare_out' } } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'filter-redirector', 'id': 'redire1', 'props': { 'netdev':
> > > > > +'hn0',
> > > > > +'queue': 'rx', 'outdev': 'compare0' } } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type': 'iothread', 'id':
> > > > > +'iothread1' } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'colo-compare',
> > > > > +'id': 'comp0', 'props': { 'primary_in': 'compare0-0', 'secondary_in':
> > > > > +'compare1', 'outdev': 'compare_out0', 'iothread': 'iothread1' }
> > > > > +} }
> > > > > +
> > > > > +{'execute': 'migrate-set-capabilities', 'arguments':{
> > > > > +'capabilities': [
> > > > > +{'capability': 'x-colo', 'state': true } ] } }
> > > > > +{'execute': 'migrate', 'arguments':{ 'uri':
> > > > > +'tcp:127.0.0.2:9998' } }
> > > > > +
> > > > > +Note:
> > > > > +If this Primary previously was a Secondary, then we need to
> > > > > +insert the filters before the filter-rewriter by using the
> > > > > +"'insert': 'before', 'position': 'id=rew0'" Options. See below.
> > > > > +
> > > > > +== Secondary resume replication == Become Primary and resume
> > > > > +replication after new Secondary is up.
> > > > > +Note that now 127.0.0.1 is the Secondary and 127.0.0.2 is the 
> > > > > Primary.
> > > > > +
> > > > > +Start the new Secondary (Steps 2 and 3 above, but with
> > > > > +primary_ip=127.0.0.2), then on the old Secondary:
> > > > > +{'execute': 'drive-mirror', 'arguments':{ 'device':
> > > > > +'colo-disk0',
> > > > > +'job-id': 'resync', 'target': 'nbd://127.0.0.1:9999/parent0', 'mode':
> > > > > +'existing', 'format': 'raw', 'sync': 'full'} }
> > > > > +
> > > > > +Wait until disk is synced, then:
> > > > > +{'execute': 'stop'}
> > > > > +{'execute': 'block-job-cancel', 'arguments':{ 'device':
> > > > > +'resync' } }
> > > > >
> > > > > -5. Failover test
> > > > > -You can kill Primary VM and run 'x_colo_lost_heartbeat' in
> > > > > Secondary VM's - monitor at the same time, then SVM will
> > > > > failover and client will not detect this -change.
> > > > > +{'execute': 'human-monitor-command', 'arguments':{ 'command-
> line':
> > > > > +'drive_add -n buddy
> > > > > +driver=replication,mode=primary,file.driver=nbd,file.host=127.0
> > > > > +.0.1 ,fil
> > > > > +e.port=9999,file.export=parent0,node-name=replication0'}}
> > > > > +{'execute': 'x-blockdev-change', 'arguments':{ 'parent':
> > > > > +'colo-disk0',
> > > > > +'node': 'replication0' } }
> > > > >
> > > > > -Before issuing '{ "execute": "x-colo-lost-heartbeat" }'
> > > > > command, we have to -issue block related command to stop block
> replication.
> > > > > -Primary:
> > > > > -  Remove the nbd child from the quorum:
> > > > > -  { 'execute': 'x-blockdev-change', 'arguments': {'parent':
> > > > > 'colo-disk0',
> > > 'child':
> > > > > 'children.1'}}
> > > > > -  { 'execute': 'human-monitor-command','arguments': {'command-
> line':
> > > > > 'drive_del blk-buddy0'}}
> > > > > -  Note: there is no qmp command to remove the blockdev now
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'filter-mirror',
> > > > > +'id': 'm0', 'props': { 'insert': 'before', 'position':
> > > > > +'id=rew0',
> > > > > +'netdev': 'hn0', 'queue': 'tx', 'outdev': 'mirror0' } } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'filter-redirector', 'id': 'redire0', 'props': { 'insert':
> > > > > +'before',
> > > > > +'position': 'id=rew0', 'netdev': 'hn0', 'queue': 'rx', 'indev':
> > > > > +'compare_out' } } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'filter-redirector', 'id': 'redire1', 'props': { 'insert':
> > > > > +'before',
> > > > > +'position': 'id=rew0', 'netdev': 'hn0', 'queue': 'rx', 'outdev':
> > > > > +'compare0' } } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type': 'iothread', 'id':
> > > > > +'iothread1' } }
> > > > > +{'execute': 'object-add', 'arguments':{ 'qom-type':
> > > > > +'colo-compare',
> > > > > +'id': 'comp0', 'props': { 'primary_in': 'compare0-0', 'secondary_in':
> > > > > +'compare1', 'outdev': 'compare_out0', 'iothread': 'iothread1' }
> > > > > +} }
> > > > >
> > > > > -Secondary:
> > > > > -  The primary host is down, so we should do the following thing:
> > > > > -  { 'execute': 'nbd-server-stop' }
> > > > > +{'execute': 'migrate-set-capabilities', 'arguments':{
> > > > > +'capabilities': [
> > > > > +{'capability': 'x-colo', 'state': true } ] } }
> > > > > +{'execute': 'migrate', 'arguments':{ 'uri':
> > > > > +'tcp:127.0.0.1:9998' } }
> > > > >
> > > > >  == TODO ==
> > > > > -1. Support continuous VM replication.
> > > > > -2. Support shared storage.
> > > > > -3. Develop the heartbeat part.
> > > > > -4. Reduce checkpoint VM’s downtime while doing checkpoint.
> > > > > +1. Support shared storage.
> > > > > +2. Develop the heartbeat part.
> > > > > +3. Reduce checkpoint VM’s downtime while doing checkpoint.