[pgpool-general: 8758] Re: Clients disconnection when slave node is off

[Jesús Campoy]

Hi Tatsuo,

I'm working with your patch but I continue facing a problem because the
database is not available during 1 second aprox (I have a script calling
select query every 0.1 seconds to check the time is not available the
database).

I will explain two different cases:

1. Slave node (backend1 in pgpool.conf) is turn off. With your patch the
database is always available. Without your patch the database is not
available during 1 second.
2. Master node (backend0) is turn off. Failover is done to promote
backend1. After that, I turn on again backend0, which is now slave node. If
I turn off this slave node (backend0), the database is not available during
1 second (with or without your patch)

Do you have any idea why is this behaviour?

Thanks in advance.

Best,
Jesús

El vie, 14 abr 2023 3:41, Tatsuo Ishii <ishii at sraoss.co.jp> escribió:

> Hi Jesús,
>
> > Hi Tatsuo,
> >
> > At first, thank you so much for your time to investigate this issue.
>
> No problem.
>
> > I have compiled pgpool 4.3.2 with your patch and the problem with pgbench
> > is solved.
> > I still need to test it in my environment.
> >
> > Anyway, I had a look your code and I have seen that the session is closed
> > only if failover is not completed in 30 seconds.
> > I have the following doubt related to this change. Is this session
> > operative during the failover? I mean, if failover spends 20 seconds, is
> > this session blocked during this time or this session can accept any
> > transaction?
>
> It is likely the session is blocked. The reason for "likely" is the
> function which has the logic inside can be called frequently during
> session but it is not always. It is possible that a pgpool process
> already called the function by the time when failover starts, then
> proceeds and sends a query to backend.
>
> > Let me another question. Should we add this issue as a bug?
>
> No you don't need. Developers already recognize this a bug report.
>
> > Thanks in advance.
> >
> > Best,
> > Jesús
> >
> >
> > El mié, 12 abr 2023 3:33, Tatsuo Ishii <ishii at sraoss.co.jp> escribió:
> >
> >> > However a downside of this is, while failover clients cannot process
> >> > queries or at least slow down processing. Below is the log from
> >> > pgbench using "-P 1" option to show progress. As you can see from 170
> >> > s pgbench starts to slow down and recovers at 194 s. That is, the
> >> > slowdown continued for 24 seconds.
> >> >
> >>
> >> After more research, I suspect the slow down is due to effect of
> >> checkpointing. If I add "-S" option to change the transaction time, I
> >> don't see the slow down anymore.
> >>
> >> Best reagards,
> >> --
> >> Tatsuo Ishii
> >> SRA OSS LLC
> >> English: http://www.sraoss.co.jp/index_en/
> >> Japanese:http://www.sraoss.co.jp
> >>
>
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