Re: [RFC][PATCH] wal decoding, attempt #2 - Design Documents (really attached)

On 10/11/2012 01:42 PM, Andres Freund wrote:
> On Thursday, October 11, 2012 09:15:47 AM Heikki Linnakangas wrote:
> ...
> If the only meaningful advantage is reducing the amount of WAL written,
> I can't help thinking that we should just try to address that in the
> existing solutions, even if it seems "easy to solve at a first glance,
> but a solution not using a normal transactional table for its log/queue
> has to solve a lot of problems", as the document says.
> Youre welcome to make suggestions, but everything I could think of that didn't
> fall short of reality ended up basically duplicating the amount of writes &
> fsyncs, even if not going through the WAL.
>
> You need to be crash safe/restartable (=> writes, fsyncs) and you need to
> reduce the writes (in memory, => !writes). There is only one authoritative
> point where you can rely on a commit to have been successfull and thats when
> the commit record has been written to the WAL. You can't send out the data to
> be committed before thats written because that could result in spuriously
> committed transactions on the remote side and you can't easily do it afterwards
> because you can crash after the commit.
Just curious here, but do you know how is this part solved in current sync
wal replication - you can get "spurious" commits on slave side id master
dies while waiting for confirmation.
> What complications are you imagining? Greetings, Andres

Hannu