[ovs-discuss] Issue with failover running ovsdb-server in A/P mode with Pacemaker

Ben Pfaff blp at ovn.org
Mon Jul 8 15:40:22 UTC 2019


ovn-controller is in fact change-aware, but the _Server database doesn't
report whether a particular database is read-only or read/write.  I
guess that was an oversight when I designed that schema.  That means
that there's no way for clients to monitor whether a particular database
changes between read-only and read/write.

I guess there are two ways to fix it:

1. Add a read/write column to the _Server schema and implement it in
   ovsdb-server and ovn-controller.

2. Make ovsdb-server kill connections when read/write status changes.

#2 is probably what we should do right away.  #1 can wait.

On Mon, Jul 08, 2019 at 01:25:09PM +0200, Daniel Alvarez Sanchez wrote:
> I *think* that it may not a bug in ovsdb-server but a problem with
> ovn-controller as it doesn't seem to be a DB change aware client.
> 
> When the role changes from master to backup or viceversa, connections
> are expected to be reestablished for all clients except those that are
> not aware of db changes [0] (note the 'false' argument). This flag is
> explained here [1] and looks like since ovn-controller is not
> monitoring the Database table in the _Server database, then the
> connection with it is not re-established. This is just a blind guess
> but  I can give it a shot :)
> 
> [0] https://github.com/openvswitch/ovs/blob/403a6a0cb003f1d48b0a3cbf11a2806c45e9d076/ovsdb/jsonrpc-server.c#L368
> [1] https://github.com/openvswitch/ovs/blob/403a6a0cb003f1d48b0a3cbf11a2806c45e9d076/ovsdb/jsonrpc-server.c#L450-L456
> 
> On Mon, Jul 8, 2019 at 12:45 PM Numan Siddique <nusiddiq at redhat.com> wrote:
> >
> >
> >
> >
> > On Mon, Jul 8, 2019 at 3:52 PM Daniel Alvarez Sanchez <dalvarez at redhat.com> wrote:
> >>
> >> Hi folks,
> >>
> >> While working with an OpenStack environment running OVN and
> >> ovsdb-server in A/P configuration with Pacemaker we hit an issue that
> >> has been probably around for a long time. The bug itself seems to be
> >> related with ovsdb-server not updating the read-only flag properly.
> >>
> >> With a 3 nodes cluster running ovsdb-server in active/passive mode,
> >> when we restart the master-node, pacemaker promotes another node as
> >> master and moves the associated IPAddr2 resource to it.
> >> At this point, ovn-controller instances across the cloud reconnect to
> >> the new node but there's a window where ovsdb-server is still running
> >> as backup.
> >>
> >> For those ovn-controller instances that reconnect within that window,
> >> every attempt to write in the OVSDB will fail with "operation not
> >> allowed when database server is in read only mode". This state will
> >> remain forever unless a reconnection is forced. Restarting
> >> ovn-controller or killing the connection (for example with tcpkill)
> >> will make things work again.
> >>
> >> A workaround in OVN OCF script could be to wait for the
> >> ovsdb_server_promote function to wait until we get 'running/active' on
> >> that instance.
> >>
> >> Another open question is what should clients (in this case,
> >> ovn-controller) do in such situation? Shall they log an error and
> >> attempt a reconnection (rate limited)?
> >
> >
> > Thanks for reporting this issue Daniel.
> >
> > I can easily  reproduce the issue with the below commands.
> >
> > $ <start the sandbox with --ovn
> > $export OVN_NB_DAEMON=$(ovn-nbctl --pidfile --detach)
> > $ovn-nbctl ls-add sw0
> > $ovs-appctl -t $PWD/sandbox/nb1 ovsdb-server/sync-status
> > state: active
> > $ovs-appctl -t $PWD/sandbox/nb1 ovsdb-server/set-active-ovsdb-server tcp:192.0.2.2:6641
> > $ovs-appctl -t $PWD/sandbox/nb1 ovsdb-server/connect-active-ovsdb-server
> > $ovs-appctl -t $PWD/sandbox/nb1 ovsdb-server/sync-status
> > state: backup
> > connecting: tcp:192.0.2.2:6641
> > $ovn-nbctl ls-add sw1  --> This should have failed. Since OVN_NB_DAEMON is set, ovn-nbctl talks to the
> >                                            ovn-nbctl daemon and it is able to create a logical switch even though the db is in backup mode
> > $unset OVN_NB_DAEMON
> > $ovn-nbctl ls-add sw2
> > ovn-nbctl: transaction error: {"details":"insert operation not allowed when database server is in read only mode","error":"not allowed"}
> >
> >
> > I looked into the ovsdb-server code, when the user changes the state of the ovsdb-server, the read_only param of  active ovsdb_server_sessions
> > are not updated.
> >
> > Thanks
> > Numan
> >
> >>
> >> Thoughts?
> >>
> >> Thanks a lot,
> >> Daniel
> >> _______________________________________________
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