Re: Issues with Quorum Commit

On 10/06/2010 09:04 PM, Dimitri Fontaine wrote:
> Ok so I think we're agreeing here: what I said amounts to propose that
> the code does work this way when the quorum is such setup, and/or is
> able to reject any non-read-only transaction (those that needs a real
> XID) until your standby is fully in sync.
>
> I'm just saying that this should be an option, not the only choice.

I'm sorry, I just don't see the use case for a mode that drops
guarantees when they are most needed. People who don't need those
guarantees should definitely go for async replication instead.

What does a synchronous replication mode that falls back to async upon
failure give you, except for a severe degradation in performance during
normal operation? Why not use async right away in such a case?

> Depends, lots of things out there work quite well in best effort mode,
> even if some projects needs more careful thinking. That's again the idea
> of waiting forever or just continuing, there's a middle-ground which is
> starting the system before reaching the durability requirements or
> downgrading it to read only, or even off, until you get them.

In such cases the admin should be free to reconfigure the quorum. And
yes, a read-only mode might be feasible. Please just don't fool the
admin with a "best effort" things that guarantees nothing (but trouble).

> If you ask for a quorum larger than what the current standbys are able
> to deliver, and you're set to wait forever until the quorum is reached,
> you just blocked the master.

Correct. That's the intended behavior.

> Good news is that the quorum is a per-transaction setting

I definitely like the per-transaction thing.

> so opening a
> superuser connection to act on the currently waiting transaction is
> still possible (pass/fail, but fail is what at this point? shutdown to
> wait some more offline?).

Not sure I'm following here. The admin will be busy re-establishing
(connections to) standbies, killing transactions on the master doesn't
help anything - whether or not the master waits forever.

Regards

Markus Wanner