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

Daniel Alvarez Sanchez dalvarez at redhat.com
Mon Jul 8 12:27:31 UTC 2019


I tried a simple patch and it fixes the issue (see below). The
question now is, do we want to do this? I think it makes sense to drop
*all* the connections when the role changes but I'm curious to see
what other people think:

diff --git a/ovsdb/jsonrpc-server.c b/ovsdb/jsonrpc-server.c
index 4dda63a..ddbbc2e 100644
--- a/ovsdb/jsonrpc-server.c
+++ b/ovsdb/jsonrpc-server.c
@@ -365,7 +365,7 @@ ovsdb_jsonrpc_server_set_read_only(struct
ovsdb_jsonrpc_server *svr,
 {
     if (svr->read_only != read_only) {
         svr->read_only = read_only;
-        ovsdb_jsonrpc_server_reconnect(svr, false,
+        ovsdb_jsonrpc_server_reconnect(svr, true,
                                        xstrdup(read_only
                                                ? "making server read-only"
                                                : "making server read/write"));


$export OVN_NB_DAEMON=$(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
ovn-nbctl: transaction error: {"details":"insert operation not allowed
when database server is in read only mode","error":"not allowed"}

On Mon, Jul 8, 2019 at 1:25 PM Daniel Alvarez Sanchez
<dalvarez at redhat.com> 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
> >> _______________________________________________
> >> discuss mailing list
> >> discuss at openvswitch.org
> >> https://mail.openvswitch.org/mailman/listinfo/ovs-discuss


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