On 6/5/24 14:54, Koen De Groote wrote:
https://www.postgresql.org/docs/current/wal-configuration.html
<https://www.postgresql.org/docs/current/wal-configuration.html>
"Checkpoints are points in the sequence of transactions at which it is
guaranteed that the heap and index data files have been updated with
all
information written before that checkpoint. At checkpoint time, all
dirty data pages are flushed to disk and a special checkpoint record is
written to the WAL file. (The change records were previously flushed to
the WAL files.) In the event of a crash, the crash recovery procedure
looks at the latest checkpoint record to determine the point in the WAL
(known as the redo record) from which it should start the REDO
operation. Any changes made to data files before that point are
guaranteed to be already on disk. Hence, after a checkpoint, WAL
segments preceding the one containing the redo record are no longer
needed and can be recycled or removed. (When WAL archiving is being
done, the WAL segments must be archived before being recycled or
removed.)"
And this is the same for logical replication and physical replication, I
take it.
High level explanation, both physical and logical replication use the
WAL files as the starting point. When the recycling is done is dependent
on various factors. My suggestion would be to read through the below to
get a better idea of what is going. There is a lot to cover, but if you
really want to understand it you will need to go through it.
Physical replication
https://www.postgresql.org/docs/current/high-availability.html
27.2.5. Streaming Replication
27.2.6. Replication Slots
Logical replication
https://www.postgresql.org/docs/current/logical-replication.html
WAL
https://www.postgresql.org/docs/current/wal.html
Thus, if a leader has a standby of the same version, and meanwhile
logical replication is being done to a newer version, both those
replications are taken into account, is that correct?
Yes, see links above.
And if it cannot sync them, due to connectivity loss for instance, the
WAL records will not be removed, then?
Depends on the type of replication being done. It is possible for
physical replication to have WAL records removed that are still needed
downstream.
From
https://www.postgresql.org/docs/current/warm-standby.html#STREAMING-REPLICATION
"If you use streaming replication without file-based continuous
archiving, the server might recycle old WAL segments before the standby
has received them. If this occurs, the standby will need to be
reinitialized from a new base backup. You can avoid this by setting
wal_keep_size to a value large enough to ensure that WAL segments are
not recycled too early, or by configuring a replication slot for the
standby. If you set up a WAL archive that's accessible from the standby,
these solutions are not required, since the standby can always use the
archive to catch up provided it retains enough segments."
This is why it is good idea to go through the links I posted above.
Regards,
Koen De Groote
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
