On Saturday, February 28, 2026 1:03 PM Amit Kapila <[email protected]> wrote: > On Fri, Feb 27, 2026 at 8:34 PM Fujii Masao <[email protected]> wrote: > > > > Normally, the slotsync worker updates the standby slot using the > > primary's slot state. However, when confirmed_flush_lsn matches but > > restart_lsn does not, the worker does not actually update the standby > > slot. Despite that, the current code of update_local_synced_slot() > > appears to treat this situation as if an update occurred. As a result, > > the worker sleeps only for the minimum interval (200 ms) before > > retrying. In the next cycle, it again assumes an update happened, and > > continues looping with the short sleep interval, causing the repeated > > logical decoding log messages. Based on a quick analysis, this seems to be > the root cause. > > > > I think update_local_synced_slot() should return false (i.e., no > > update > > happened) when confirmed_flush_lsn is equal but restart_lsn differs > > between primary and standby. > > > > We expect that in such a case update_local_synced_slot() should advance > local_slot's 'restart_lsn' via LogicalSlotAdvanceAndCheckSnapState(), > otherwise, it won't go in the cheap code path next time. Normally, restart_lsn > advancement should happen when we process XLOG_RUNNING_XACTS and > call SnapBuildProcessRunningXacts(). In this particular case as both > restart_lsn and confirmed_flush_lsn are the same (0/03000140), the > machinery may not be processing XLOG_RUNNING_XACTS record. I have not > debugged the exact case yet but you can try by emitting some more records > on publisher, it should let the standby advance the slot. It is possible that > we > can do something like you are proposing to silence the LOG messages but we > should know what is going on here.
I reproduced and debugged this issue where a replication slot's restart_lsn
fails to advance. In my environment, I found it only occurs when a synced
slot first builds a consistent snapshot. The problematic code path is in
SnapBuildProcessRunningXacts():
if (builder->state < SNAPBUILD_CONSISTENT)
{
/* returns false if there's no point in performing cleanup just yet */
if (!SnapBuildFindSnapshot(builder, lsn, running))
return;
}
When a synced slot reaches consistency for the first time with no running
transactions, SnapBuildFindSnapshot() returns false, causing the function to
return without updating the candidate restart_lsn.
So, an alternative approach is to improve this logic by updating the candidate
restart_lsn in this case instead of returning early. See the attached patch for
details. This can fix the issue on my machine.
Best Regards,
Hou zj
v1-0001-Advance-restart_lsn-when-reaching-consistency-wit.patch
Description: v1-0001-Advance-restart_lsn-when-reaching-consistency-wit.patch
