Hi,
On Thu, Mar 28, 2024 at 04:38:19AM +0000, Zhijie Hou (Fujitsu) wrote:
> Hi,
>
> When analyzing one BF error[1], we find an issue of slotsync: Since we don't
> perform logical decoding for the synced slots when syncing the lsn/xmin of
> slot, no logical snapshots will be serialized to disk. So, when user starts to
> use these synced slots after promotion, it needs to re-build the consistent
> snapshot from the restart_lsn if the WAL(xl_running_xacts) at restart_lsn
> position indicates that there are running transactions. This however could
> cause the data that before the consistent point to be missed[2].
I see, nice catch and explanation, thanks!
> This issue doesn't exist on the primary because the snapshot at restart_lsn
> should have been serialized to disk (SnapBuildProcessRunningXacts ->
> SnapBuildSerialize), so even if the logical decoding restarts, it can find
> consistent snapshot immediately at restart_lsn.
Right.
> To fix this, we could use the fast forward logical decoding to advance the synced
> slot's lsn/xmin when syncing these values instead of directly updating the
> slot's info. This way, the snapshot will be serialized to disk when decoding.
> If we could not reach to the consistent point at the remote restart_lsn, the
> slot is marked as temp and will be persisted once it reaches the consistent
> point. I am still analyzing the fix and will share once ready.
Thanks! I'm wondering about the performance impact (even in fast_forward mode),
might be worth to keep an eye on it.
Should we create a 17 open item [1]?
[1]: https://wiki.postgresql.org/wiki/PostgreSQL_17_Open_Items
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
