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

On Sun, Nov 8, 2015 at 9:50 PM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> On Sat, Nov 7, 2015 at 3:54 PM, Michael Paquier wrote:
>> I thought about something like that at some point by saving a minimum
>> activity pointer in XLogCtl, updated each time a segment was forcibly
>> switched or after inserting a checkpoint record. Then the bgwriter
>> looked at if the current insert position matched this minimum activity
>> pointer, skipping LogStandbySnapshot if both positions match. Does
>> this match your line of thoughts?
>
> Looking at the code, it occurred to me that the LSN position saved for
> a XLOG_SWITCH record is the last position of current segment, so we
> would still need to check if the current insert LSN matches the
> beginning of a new segment and if the last segment was forcibly
> switched by saving RecPtr of RequestXLogSwitch in XLogCtl for example.
> Thoughts?

I haven't given up on this patch yet, and putting again my head on
this problem I have finished with the patch attached, which checks if
the current insert LSN position is at the beginning of a segment that
has just been switched to decide if a standby snapshot should be
logged or not. This allows bringing back an idle system to the pre-9.3
state where a segment would be archived in the case of a low
archive_timeout only when a checkpoint has been issued on the system.
In order to achieve this idea I have added a field on XLogCtl that
saves the last time a segment has been forcibly switched after
XLOG_SWITCH.

Honestly I am failing to see why we should track the progress since
last checkpoint as mentioned upthread, and the current behavior is
certainly a regression.

Speaking of which, this patch was registered in this CF, I am moving
it to the next as a bug fix.
Regards,
--
Michael