On Fri, 2023-11-24 at 16:59 +0100, Les wrote: > > > Laurenz Albe <laurenz.albe@xxxxxxxxxxx> (2023. nov. 24., P, 16:00): > > On Fri, 2023-11-24 at 12:39 +0100, Les wrote: > > > Under normal circumstances, the number of write operations is relatively low, with an > > > average of 4-5 MB/sec total write speed on the disk associated with the data directory. > > > Yesterday, the primary server suddenly started writing to the pg_wal directory at a > > > crazy pace, 1.5GB/sec, but sometimes it went up to over 3GB/sec. > > > [...] > > > Upon further analysis of the database, we found that we did not see any mass data > > > changes in any of the tables. The only exception is a sequence value that was moved > > > millions of steps within a single minute. > > > > That looks like some application went crazy and inserted millions of rows, but the > > inserts were rolled back. But it is hard to be certain with the clues given. > > Writing of WAL files continued after we shut down all clients, and restarted the primary PostgreSQL server. > > How can the primary server generate more and more WAL files (writes) after all clients have > been shut down and the server was restarted? My only bet was the autovacuum. But I ruled > that out, because removing a replication slot has no effect on the autovacuum (am I wrong?). It must have been autovacuum. Removing a replication slot has an influence, since then autovacuum can do more work. If the problem stopped when you dropped the replication slot, it could be a coincidence. > Now you are saying that this looks like a huge rollback. It could have been many small rollbacks. > Does rolling back changes require even more data to be written to the WAL after server > restart? No. My assumption would be that something generated lots of INSERTs that were all rolled back. That creates WAL, even though you see no change in the table data. > Does removing a replication slot lessen the amount of data needed to be written for > a rollback (or for anything else)? No: the WAL is generated by whatever precedes the ROLLBACK, and the ROLLBACK does not create a lot of WAL. > It is a fact that the primary stopped writing at 1.5GB/sec the moment we removed the slot. I have no explanation for that, except a coincidence. Replication slots don't generate WAL. Yours, Laurenz Albe
