On 2020-09-17 15:27, Alexander Kukushkin wrote:
On Thu, 17 Sep 2020 at 14:04, Alexey Kondratov
<a.kondra...@postgrespro.ru> wrote:
With --restore-target-wal pg_rewind is trying to call restore_command
on
its own and it can happen at two stages:
1) When pg_rewind is trying to find the last checkpoint preceding a
divergence point. In that case file map is not even yet initialized.
Thus, all fetched WAL segments at this stage will be present in the
file
map created later.
Nope, it will fetch files you requested, and in addition to that it
will leave a child process running in the background which is doing
the prefetch (manipulating with pg_wal/.wal-g/...)
2) When it creates a data pages map. It should traverse WAL from the
last common checkpoint till the final shutdown point in order to find
all modified pages on the target. At this stage pg_rewind only updates
info about data segments in the file map. That way, I see a minor
problem that WAL segments fetched at this stage would not be deleted,
since they are absent in the file map.
Anyway, pg_rewind does not delete neither WAL segments, not any other
files in the middle of the file map creation, so I cannot imagine, how
it can get into the same trouble on its own.
When pg_rewind was creating the map, some temporary files where there,
because the forked child process of wal-g was still running.
When the wal-g child process exits, it removes some of these files.
Specifically, it was trying to prefetch 0000008400000A7600000024 into
the pg_wal/.wal-g/prefetch/running/0000008400000A7600000024, but
apparently the file wasn't available on S3 and prefetch failed,
therefore the empty file was removed.
I do understand how you got into this problem with wal-g. This part of
my answer was about bare postgres and pg_rewind. And my point was that
from my perspective pg_rewind with --restore-target-wal cannot get into
the same trouble on its own, without 'help' of some side tools like
wal-g.
Regards
--
Alexey Kondratov
Postgres Professional https://www.postgrespro.com
Russian Postgres Company
