Re: [REVIEW] pg_last_xact_insert_timestamp

On Mon, Dec 12, 2011 at 12:17 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On Sat, Dec 10, 2011 at 12:29 PM, Greg Smith <greg@2ndquadrant.com> wrote:
>
>> "We can send regular special messages from WALSender to WALReceiver that do
>> not form part of the WAL stream, so we don't bulk
>> up WAL archives. (i.e. don't use "w" messages)."
>>
>> Here's my understanding of how this would work.
>
> Let me explain a little more and provide a very partial patch.
>
> We define a new replication protocol message 'k' which sends a
> keepalive from primary to standby when there is no WAL to send. The
> message does not form part of the WAL stream so does not bloat WAL
> files, nor cause them to fill when unattended.
>
> Keepalives contain current end of WAL and a current timestamp.
>
> Keepalive processing is all done on the standby and there is no
> overhead on a primary which does not use replication. There is a
> slight overhead on primary for keepalives but this happens only when
> there are no writes. On the standby we already update shared state
> when we receive some data, so not much else to do there.
>
> When the standby has applied up to the end of WAL the replication
> delay is receipt time - send time of keepalive.

Patch introduces regular keepalives from WALsender to WALreceiver,
using a new protocol message 'k'.
These are sent at intervals of a fraction of replication_delay or 10s
if not set,
whenever no WAL records have been sent recently.

Patch exposes info used for standby_delay calculation as used in 9.0+
In addition introduces direct calculations of replication apply delay
and replication transfer latency, both in ms.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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