[ovs-dev] [PATCH ovn 1/2] Revert "northd: Don't poll ovsdb before the connection is fully established"

[Han Zhou]

On Fri, Sep 17, 2021 at 2:04 PM Numan Siddique <nusiddiq at redhat.com> wrote:
>
> On Fri, Sep 17, 2021 at 5:02 PM Renat Nurgaliyev <impleman at gmail.com>
wrote:
> >
> > Hi Han,
> >
> > yes, I believe you are totally right. But it still feels like a chicken
and
> > egg problem to me, storing the database timeout setting inside the
database
> > itself. If there would be at least some local command line argument to
> > override timeout value, it would be already amazing, because currently
> > there is no way to control it before the database connection is made,
and
> > if it cannot be made, it is too late to try to control it.
> >
>
> What about the case where the NB database is huge and it takes > 5
> seconds to fetch
> all the contents ?
>
I think Renat had the answer to this question: using the DB to configure
the probe interval to the DB is going to be a problem in certain cases. It
should be fine to use NB to configure probe interval for SB, but using NB
to configure the probe interval for NB itself is definitely causing the
problem when the NB is huge. Support command line options may be the right
approach. But in practice would it be good enough to use 60s as the default
value instead of 5s?

> Numan
>
> > Thanks,
> > Renat.
> >
> > Han Zhou <hzhou at ovn.org> schrieb am Fr., 17. Sept. 2021, 23:55:
> >
> > >
> > >
> > > On Fri, Sep 17, 2021 at 1:48 PM Renat Nurgaliyev <impleman at gmail.com>
> > > wrote:
> > > >
> > > > Hello Han,
> > > >
> > > > when I wrote this patch we had an issue with a very big SB database,
> > > around 1,5 gigabytes. There were no controllers or northds running,
so the
> > > database server was without any load at all. Although OVSDB was
idling,
> > > even a single northd process could not fully connect to the database
due to
> > > its size, since it could not fetch and process the data in 5 seconds.
> > >
> > > Hi Renat, thanks for the explanation. However, suppose SB is still
huge,
> > > if NB is not that big, the probe config in NB_Global will soon be
applied
> > > to ovn-northd, which would probe in proper interval (desired setting
with
> > > the SB size considered) instead of the default 5 sec, and it should
> > > succeed, right?
> > >
> > > Thanks,
> > > Han
> > >
> > > >
> > > > Since then many optimizations were made, and the database size with
the
> > > same topology reduced to approximately twenty megabytes, so today I
> > > wouldn't be able to reproduce the problem.
> > > >
> > > > However, I am quite sure that it would still cause troubles with a
huge
> > > scale, when SB grows to hundreds of megabytes. With the default
timeout of
> > > 5 seconds, which is implemented in the same thread that also fetches
and
> > > processes data, we make an artificial database size limit, which is
not so
> > > obvoius to troubleshoot.
> > > >
> > > > Regards,
> > > > Renat.
> > > >
> > > > Han Zhou <hzhou at ovn.org> schrieb am Fr., 17. Sept. 2021, 23:34:
> > > >>
> > > >>
> > > >>
> > > >> On Thu, Sep 16, 2021 at 8:05 PM Zhen Wang <zhewang at nvidia.com>
wrote:
> > > >> >
> > > >> > From: zhen wang <zhewang at nvidia.com>
> > > >> >
> > > >> > This reverts commit 1e59feea933610b28fd4442243162ce35595cfee.
> > > >> > Above commit introduced a bug when muptiple ovn-northd instances
work
> > > in HA
> > > >> > mode. If SB leader and active ovn-northd instance got killed by
> > > system power
> > > >> > outage, standby ovn-northd instance would never detect the
failure.
> > > >> >
> > > >>
> > > >> Thanks Zhen! I added the Renat and Numan who worked on the reverted
> > > commit to CC, so that they can comment if this is ok.
> > > >>
> > > >> For the commit message, I think it may be decoupled from the HA
> > > scenario that is supposed to be fixed by the other patch in this
series.
> > > The issue this patch fixes is that before the initial NB downloading
is
> > > complete the northd will not send probe, so if the DB server is down
> > > (ungracefully) before the northd reads the NB_Global options, the
northd
> > > would never probe, thus never reconnect to the new leader. (it is
related
> > > to RAFT, but whether it is multiple northds is irrelevant)
> > > >>
> > > >> As to the original commit that is reverted by this one:
> > > >>
> > > >>     northd: Don't poll ovsdb before the connection is fully
established
> > > >>
> > > >>     Set initial SB and NB DBs probe interval to 0 to avoid
connection
> > > >>     flapping.
> > > >>
> > > >>     Before configured in northd_probe_interval value is actually
applied
> > > >>     to southbound and northbound database connections, both
connections
> > > >>     must be fully established, otherwise ovnnb_db_run() will return
> > > >>     without retrieving configuration data from northbound DB. In
cases
> > > >>     when southbound database is big enough, default interval of 5
> > > seconds
> > > >>     will kill and retry the connection before it is fully
established,
> > > no
> > > >>     matter what is set in northd_probe_interval. Client reconnect
will
> > > >>     cause even more load to ovsdb-server and cause cascade effect,
so
> > > >>     northd can never stabilise. We have more than 2000 ports in
our lab,
> > > >>     and northd could not start before this patch, holding at 100%
CPU
> > > >>     utilisation both itself and ovsdb-server.
> > > >>
> > > >>     After connections are established, any value in
> > > northd_probe_interval,
> > > >>     or default DEFAULT_PROBE_INTERVAL_MSEC is applied correctly.
> > > >>
> > > >> I am not sure how would the commit help. There are at most 3 - 5
> > > northds (in practice), and suppose there are tens or hundreds of
> > > ovn-controllers that makes SB busy, it is just 3 - 5 more clients
retrying
> > > reconnect SB for several times, and if NB is not that busy (most
likely),
> > > these northd clients should get the proper probe settings applied soon
> > > without causing more issues at all. So I don't think the default
probe 5
> > > sec would cause cascade effect for the initial period. @Renat @Numan
please
> > > correct me if I am wrong.
> > > >>
> > > >> Thanks,
> > > >> Han
> > > >>
> > > >> > Signed-off-by: zhen wang <zhewang at nvidia.com>
> > > >> > ---
> > > >> >  northd/northd.c | 4 ++--
> > > >> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >> >
> > > >> > diff --git a/northd/northd.c b/northd/northd.c
> > > >> > index 688a6e4ef..b7e64470f 100644
> > > >> > --- a/northd/northd.c
> > > >> > +++ b/northd/northd.c
> > > >> > @@ -74,8 +74,8 @@ static bool use_ct_inv_match = true;
> > > >> >
> > > >> >  /* Default probe interval for NB and SB DB connections. */
> > > >> >  #define DEFAULT_PROBE_INTERVAL_MSEC 5000
> > > >> > -static int northd_probe_interval_nb = 0;
> > > >> > -static int northd_probe_interval_sb = 0;
> > > >> > +static int northd_probe_interval_nb =
DEFAULT_PROBE_INTERVAL_MSEC;
> > > >> > +static int northd_probe_interval_sb =
DEFAULT_PROBE_INTERVAL_MSEC;
> > > >> >  #define MAX_OVN_TAGS 4096
> > > >> >
> > > >> >  /* Pipeline stages. */
> > > >> > --
> > > >> > 2.20.1
> > > >> >
> > >
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>