Re: cheaper snapshots

On Thu, 2011-07-28 at 20:14 -0400, Robert Haas wrote:
> On Thu, Jul 28, 2011 at 7:54 PM, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> wrote:
> > On Thu, Jul 28, 2011 at 11:54 PM, Kevin Grittner
> > <Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> >> (4) We communicate acceptable snapshots to the replica to make the
> >> order of visibility visibility match the master even when that
> >> doesn't match the order that transactions returned from commit.
> >
> > I wonder if some interpretation of 2 phase commit could make Robert's
> > original suggestion implement this.
> >
> > On the master the commit sequence would look something like:
> > 1. Insert commit record to the WAL
> > 2. Wait for replication
> > 3. Get a commit seq nr and mark XIDs visible
> > 4. WAL log the seq nr
> > 5. Return success to client
> >
> > When replaying:
> > * When replaying commit record, do everything but make
> > the tx visible.
> > * When replaying the commit sequence number
> > if there is a gap between last visible commit and current:
> > insert the commit sequence nr. to list of waiting commits.
> > else:
> > mark current and all directly following waiting tx's visible
> >
> > This would give consistent visibility order on master and slave. Robert
> > is right that this would undesirably increase WAL traffic. Delaying this
> > traffic would undesirably increase replay lag between master and slave.
> > But it seems to me that this could be an optional WAL level on top of
> > hot_standby that would only be enabled if consistent visibility on
> > slaves is desired.
>
> I think you nailed it.

Agreed, this would keep current semantics on master and same visibility
order on master and slave.

> An additional point to think about: if we were willing to insist on
> streaming replication, we could send the commit sequence numbers via a
> side channel rather than writing them to WAL, which would be a lot
> cheaper.

Why do you think that side channel is cheaper than main WAL ?

How would you handle synchronising the two ?

> That might even be a reasonable thing to do, because if
> you're doing log shipping, this is all going to be super-not-real-time
> anyway.

But perhaps you still may want to preserve visibility order to be able
to do PITR to exact transaction "commit", no ?

> OTOH, I know we don't want to make WAL shipping anything less
> than a first class citizen, so maybe not.
>
> At any rate, we may be getting a little sidetracked here from the
> original point of the thread, which was how to make snapshot-taking
> cheaper. Maybe there's some tie-in to when transactions become
> visible, but I think it's pretty weak. The existing system could be
> hacked up to avoid making transactions visible out of LSN order, and
> the system I proposed could make them visible either in LSN order or
> do the same thing we do now. They are basically independent problems,
> AFAICS.

Agreed.

--
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Hannu Krosing
PostgreSQL Infinite Scalability and Performance Consultant
PG Admin Book: http://www.2ndQuadrant.com/books/