Horton

Re: Restricting maximum keep segments by repslots

Craig Ringer <craig@2ndquadrant.com>

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From: Craig Ringer <craig@2ndquadrant.com>
To: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2017-03-07T02:01:53Z
Lists: pgsql-hackers 
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

Commits

  1. Save slot's restart_lsn when invalidated due to size

  2. Fix checkpoint signalling

  3. Check slot->restart_lsn validity in a few more places

  4. Allow users to limit storage reserved by replication slots

  5. Remove header noise from test_decoding test

  6. Rework WAL-reading supporting structs

  7. Flip argument order in XLogSegNoOffsetToRecPtr