Re: checkpoints are duplicated even while the system is idle

On Thu, Oct 6, 2011 at 2:26 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, Oct 6, 2011 at 1:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I'm inclined to think that the way to deal with that is not to force out
>>> useless WAL data, but to add some sort of explicit "I'm alive" heartbeat
>>> signal to the walsender/walreceiver protocol.  The hard part of that is
>>> to figure out how to expose it where you can see it on the slave side
>>> --- or do we have a status view that could handle that?
>
>> As of 9.1, we already have something very much like this, in the
>> opposite direction.  See wal_receiver_status_interval and
>> replication_timeout.  I bet we could adapt that slightly to work in
>> the other direction, too.  But that'll only work with streaming
>> replication - do we care about the WAL shipping case?
>
> I can't get excited about the WAL-shipping case. The artifact that we'll
> generate a checkpoint record every few minutes does not create enough
> WAL volume for WAL-shipping to reliably generate a heartbeat signal.
> It'd be way too long between filling up segments if that were the only
> WAL data being generated.

Depends how you set archive_timeout, I think.

Anyway, I'm not saying we *have* to care about that case. I'm just
asking whether we do, before we go start changing things.

--
Robert Haas
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