Re: POC: Cleaning up orphaned files using undo logs

On Mon, Sep 27, 2021 at 7:43 PM Antonin Houska <ah(at)cybertec(dot)at> wrote:
>
> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Although the postgres core probably does not raise FATAL errors too often (OOM
> conditions seem to be the typical cause), I'm still not enthusiastic about
> idea that the undo feature turns such errors into PANIC.
>
> I wonder what the reason to avoid undoing transaction on FATAL is. If it's
> about possibly long duration of the undo execution, deletion of orphaned files
> (relations or the whole databases) via undo shouldn't make things worse
> because currently FATAL also triggers this sort of cleanup immediately, it's
> just implemented in different ways.
>

During FATAL, we don't want to perform more operations which can make
the situation worse. Say, we are already short of memory (OOM), and
undo execution can further try to allocate the memory won't do any
good. Depending on the implementation, sometimes undo execution might
need to perform WAL writes or data write which we don't want to do
during FATAL error processing.

--
With Regards,
Amit Kapila.