Re: [HACKERS] Restricting maximum keep segments by repslots

On 28 February 2017 at 12:27, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote:

>> This patch adds a GUC to put a limit to the number of segments
>> that replication slots can keep. Hitting the limit during
>> checkpoint shows a warining and the segments older than the limit
>> are removed.
>>
>>> WARNING:  restart LSN of replication slots is ignored by checkpoint
>>> DETAIL:  Some replication slots lose required WAL segnents to continue.
>>
>
> However this is dangerous as logical replication slot does not consider
> it error when too old LSN is requested so we'd continue replication,
> hiding data loss.

That skipping only happens if you request a startpoint older than
confirmed_flush_lsn . It doesn't apply to this situation.

The client cannot control where we start decoding, it's always
restart_lsn, and if we can't find a needed WAL segment we'll ERROR. So
this is safe, though the error will be something about being unable to
find a wal segment that users might not directly associate with having
set this option. It won't say "slot disabled because needed WAL has
been discarded due to [setting]" or anything.



-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services