Re: [Qemu-block] [PATCH RFC v2 1/6] docs/block-replication: Add description for shared-disk case

[Hailiang Zhang]

On 2017/1/13 21:41, Stefan Hajnoczi wrote:
> On Mon, Dec 05, 2016 at 04:34:59PM +0800, zhanghailiang wrote:
> > +Issue qmp command:
> > +  { 'execute': 'blockdev-add',
> > +    'arguments': {
> > +        'driver': 'replication',
> > +        'node-name': 'rep',
> > +        'mode': 'primary',
> > +        'shared-disk-id': 'primary_disk0',
> > +        'shared-disk': true,
> > +        'file': {
> > +            'driver': 'nbd',
> > +            'export': 'hidden_disk0',
> > +            'server': {
> > +                'type': 'inet',
> > +                'data': {
> > +                    'host': 'xxx.xxx.xxx.xxx',
> > +                    'port': 'yyy'
> > +                }
> > +            }
>
> block/nbd.c does have good error handling and recovery in case there is
> a network issue.  There are no reconnection attempts or timeouts that
> deal with a temporary loss of network connectivity.
>
> This is a general problem with block/nbd.c and not something to solve in
> this patch series.  I'm just mentioning it because it may affect COLO
> replication.
>
> I'm sure these limitations in block/nbd.c can be fixed but it will take
> some effort.  Maybe block/sheepdog.c, net/socket.c, and other network
> code could also benefit from generic network connection recovery.

Hmm, good suggestion, but IMHO, here, COLO is a little different from
other scenes, if the reconnection method has been implemented,
it still needs a mechanism to identify the temporary loss of network
connection or real broken in network connection.

I did a simple test, just ifconfig down the network card that be used
by block replication, It seems that NBD in qemu doesn't has a ability to
find the connection has been broken, there was no error reports
and COLO just got stuck in vm_stop() where it called aio_poll().

Thanks,
Hailiang



> Reviewed-by: Stefan Hajnoczi <address@hidden>