Re: Can there ever be out of sequence WAL files?

On Wed, Jan 12, 2022 at 10:18:11AM +0800, Julien Rouhaud wrote:
> On Wed, Jan 12, 2022 at 07:19:48AM +0530, Bharath Rupireddy wrote:
>>> Can the postgres server ever have/generate out of sequence WAL files?
>>> For instance, 000000010000020C000000A2, 000000010000020C000000A3,
>>> 000000010000020C000000A5 and so on, missing 000000010000020C000000A4.
>>> Manual/Accidental deletion of the WAL files can happes, but are there
>>> any other extreme situations (like recycling, removing old WAL files
>>> etc.) caused by the postgres server leading to missing WAL files?
>
> By definition there shouldn't be such situation, as it would otherwise be a
> (critical) bug.

I have seen that in the past, in cases where a system got harshly
deplugged then replugged where a segment file flush got missing. But
that was just a flacky system, Postgres relied just on something
wrong. So the answer is that this should not happen.

>>> What happens when postgres server finds missing WAL file during
>>> crash/standby recovery?
>
> The recovery should fail.

xlog.c can be a good read to check the assumptions WAL replay relies
on, with things like CheckRecoveryConsistency() or
reachedConsistency.
--
Michael