Thread

Re: Making pg_rewind faster

Srinath Reddy Sadipiralla <srinath2133@gmail.com> — 2025-10-09T19:09:43Z

Hi,

just a second late :( i was about to post a patch addressing the refactors
which Robert mentioned  ,anyway will have a look at your latest patch John
thanks :), curious about the tap test.

while i was writing the patch something suddenly struck me , that is why we
are even depending on last_common_segno ,because once we
reached decide_wal_file_action it means that the file exists in both target
and source ,AFAIK this can only happen with wal segments older than or
equal to last_common_segno because once the promotion competes the filename
of the WAL files gets changed with the new timelineID(2), for ex: if the
last_common_segno is 000000010000000000000003 then based on the rules
in XLogInitNewTimeline
1) if the timeline switch happens in middle of segment ,copy data from the
last WAL segment and create WAL file with same segno but different
timelineID,in this case the starting WAL file for the new timeline will be
000000020000000000000003
2) if the timeline switch happens at segment boundary , just create next
segment for this case the starting WAL file for the new timeline will be
000000020000000000000004

so basically the files which exists in source and not in target like the
new timeline WAL segments will be copied to target in total before we reach
decide_wal_file_action , so i think we don't need to think about copying
WAL files after divergence point by calculating and checking against
last_common_segno which we are doing in our current approach , i think we
can just do

in decide_wal_file_action

if (entry->target_size != entry->source_size)
{
pg_log_debug("WAL segment file \"%s\" is copied to target", fname);
return FILE_ACTION_COPY;
}
return FILE_ACTION_NONE;

thoughts?

-- 
Thanks,
Srinath Reddy Sadipiralla
EDB: https://www.enterprisedb.com/