Re: Re: BUG #13685: Archiving while idle every archive_timeout with wal_level hot_standby

On Wed, Nov 4, 2015 at 12:43 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2015-11-03 10:23:35 -0500, Robert Haas wrote:
>> On Mon, Nov 2, 2015 at 12:58 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> > If a transaction holding locks aborts on an otherwise idle server, perhaps it will take a very long time for a log-shipping standby to realize this. But I have hard time believing that anyone who cares about that would be using log-shipping (rather than streaming) anyway.
>>
>> I'm sure other people here understand this better than me, but I
>> wonder if it wouldn't make more sense to somehow log this data only if
>> something material has changed in the data being logged.
>
> Phew. That doesn't seem easy to measure. I'm doubtful that it's worth
> comparing the snapshot and such, especially in the back
> branches.

Well, I guess that's why I thought it would be more simple to check if
we are at the beginning of a segment at first sight. This has no
chance to break if anything else like that is being added in the
future as it doesn't depend on the record types, though new similar
records added on a timely manner would need a similar check. Perhaps
this could be coupled by a check on the last XLOG_SWITCH_XLOG record
instead of checkpoint activity though.

> We could maybe add something that we only log a snapshot if XXX
> megabytes have been logged or something. But I don't know which number
> to pick here - and if there's other write activity the price of a
> snapshot record really isn't high.

On a completely idle system, I don't think we should log any standby
records. This is what ~9.3 does.
--
Michael