Re: [Qemu-devel] [PATCH v5 0/4] qapi: child add/delete support

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wen Congyang (wency@cn.fujitsu.com) wrote:
> On 09/22/2015 07:15 PM, Dr. David Alan Gilbert wrote:
> > * Wen Congyang (wency@cn.fujitsu.com) wrote:
> >> If quorum's child is broken, we can use mirror job to replace it.
> >> But sometimes, the user only need to remove the broken child, and
> >> add it later when the problem is fixed.
> >>
> > 
> > Hi,
> >   Two questions:
> >     1) Do you have an example of a pair of add/remove commands that work
> >       together? (I'm not quite sure I understand where the ID for the remove
> >       comes from).
> 
> The command line:
> -drive if=virtio,id=disk1,driver=quorum,read-pattern=fifo,vote-threshold=1,children.0.file.filename=/data/images/kvm/suse/suse11_3.img,children.0.driver=raw
> 
> And the QMP monitor command:
> {'execute':'blockdev-add', 'arguments':{'options':{'driver': 'raw', 'node-name': 'test1', 'file': {'driver': 'file', 'filename': '/dev/null'}, 'id': 'test11' }  } }
> {'execute': 'human-monitor-command', 'arguments': {'command-line': 'drive_add buddy driver=nbd,host=192.168.3.1,port=8889,export=colo-disk1,node-name=test2,if=none'}}
> {'execute':'x-blockdev-child-add', 'arguments':{'parent': 'disk1', 'child': 'test1' } }
> {'execute':'x-blockdev-child-add', 'arguments':{'parent': 'disk1', 'child': 'test2' } }
> {'execute': 'x-blockdev-child-del', 'arguments': {'parent': 'disk1', 'child': 'test1' } }
> {'execute': 'x-blockdev-child-del', 'arguments': {'parent': 'disk1', 'child': 'test2' } }
> 
> Note: the qmp monitor command doesn't support nbd now, and I use the hmp command to add a BDS.

Thank you; OK I see the format has changed quite a bit from the older version; this version
is a lot nicer.

> >     2) If the child has failed and is not responding to block operations
> >        at all (e.g a networking failure to an nbd device which may take minutes
> >        to time out); how do you recover - flush or drain on the devices
> >        hang at that point.
> 
> If the network fails, the kernel doesn't notify the application...
> 
> > 
> > (I was trying to test recovery from a failed secondary using the July COLO
> > release; but the primary gets stuck in bdrv_drain or bdrv_flush if I kill
> > the secondary in the right way).
> 
> IIRC, if the qemu is killed, the connection is closed at the same time. bdrv_drain()
> or bdrv_flush() should not get stuck.

I use kill -SIGSTOP to the secondary qemu so I think that behaves like the network fails,
or if the secondary host just failed completely.  You do need some way to recover from the
NBD server dieing like that.

It sounds like we need some way to be able to remove a blockdev that's failed like that;
Paolo suggested the 'disk deadline' series could be used to time something like that
out eventually, but maybe you need something that allows you to remove
a child more forcibly.

Dave

> 
> Thanks
> Wen Congyang
> 
> > 
> > Dave
> > 
> > 
> >> It is based on the following patch:
> >> http://lists.nongnu.org/archive/html/qemu-devel/2015-09/msg04579.html
> >>
> >> ChangLog:
> >> v5:
> >> 1. Address Eric Blake's comments
> >> v4:
> >> 1. drop nbd driver's implementation. We can use human-monitor-command
> >>    to do it.
> >> 2. Rename the command name.
> >> v3:
> >> 1. Don't open BDS in bdrv_add_child(). Use the existing BDS which is
> >>    created by the QMP command blockdev-add.
> >> 2. The driver NBD can support filename, path, host:port now.
> >> v2:
> >> 1. Use bdrv_get_device_or_node_name() instead of new function
> >>    bdrv_get_id_or_node_name()
> >> 2. Update the error message
> >> 3. Update the documents in block-core.json
> >>
> >> Wen Congyang (4):
> >>   Add new block driver interface to add/delete a BDS's child
> >>   quorum: implement bdrv_add_child() and bdrv_del_child()
> >>   qmp: add monitor command to add/remove a child
> >>   hmp: add monitor command to add/remove a child
> >>
> >>  block.c                   | 56 ++++++++++++++++++++++++++++++++++--
> >>  block/quorum.c            | 72 +++++++++++++++++++++++++++++++++++++++++++++--
> >>  blockdev.c                | 48 +++++++++++++++++++++++++++++++
> >>  hmp-commands.hx           | 28 ++++++++++++++++++
> >>  hmp.c                     | 20 +++++++++++++
> >>  hmp.h                     |  2 ++
> >>  include/block/block.h     |  8 ++++++
> >>  include/block/block_int.h |  5 ++++
> >>  qapi/block-core.json      | 34 ++++++++++++++++++++++
> >>  qmp-commands.hx           | 61 +++++++++++++++++++++++++++++++++++++++
> >>  10 files changed, 329 insertions(+), 5 deletions(-)
> >>
> >> -- 
> >> 2.4.3
> >>
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > .
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK