Hello, John. At Fri, 10 Jul 2020 20:44:30 +0000, "Hsu, John" <hsuc...@amazon.com> wrote in > Hi hackers, > > We believe we’re seeing a problem with how physical slot’s restart_lsn is > advanced leading to the replicas needing to restore from archive in order for > replication to resume. > The logs below are from reproductions against 10.13. I’m still working on > reproducing it for 12.3. > > WAL write spans two WAL segments . > Write to first WAL segment is complete but not to the second segment. > Write to first WAL segment is acknowledged as flushed from the Postgres > replica. > Primary restarts before the write to second segment completes. It also means > the complete WAL was never written. > Crash recovery finishes at a record before the incomplete WAL write. > Though now replica start the slot at the next WAL segment, since the previous > WAL was already acknowledged as flushed. ... > Redo finishes at 0/2BFFFFB0 even though the flush we received from > the replica is already at 0/2C000000. > This is problematic because the replica reconnects to the slot > telling it to start past the new redo point. Yeah, that is a problem not only related to restart_lsn. The same cause leads to aother issue of inconsistent archive as discussed in [1].
1: https://www.postgresql.org/message-id/CBDDFA01-6E40-46BB-9F98-9340F4379505%40amazon.com > The attached patch (against 10) attempts to address this by keeping > track of the first flushLsn in the current segNo, and wait until we > receive one after that before updating. This prevents the WAL from > rotating out of the primary and a reboot from the replica will fix > it instead of needing to restore from archive. On the other hand we can and should advance restart_lsn when we know that the last record is complete. I think a patch in the thread [2] would fix your issue. With the patch primary doesn't send a continuation record at the end of a segment until the whole record is flushed into WAL file. 2: https://www.postgresql.org/message-id/20200625.153532.379700510444980240.horikyota.ntt%40gmail.com regards. -- Kyotaro Horiguchi NTT Open Source Software Center
