Re: False "pg_serial": apparent wraparound” in logs

On Sat, Oct 14, 2023 at 07:29:54PM +0000, Imseih (AWS), Sami wrote:
>> Anyway, it looks like you're right, we don't really need the SLRU once
>> the tail is ahead of the tail because the SLRU has wrapped around due
>> to the effect of transactions aging out, so making the truncation a
>> bit smarter should be OK.
>
> I assume you meant " the tail is ahead of the head".

Damn fingers on a keyboard who don't know how to type.

>> Hmm. This doesn't seem enough. Shouldn't we explain at least in
>> which scenarios the tail can get ahead of the head (aka at least
>> with long running transactions that make the SLRU wrap-around)?
>> Except if I am missing something, there is no explanation of that in
>> predicate.c.
>
> After looking at this a bit more, I don't think the previous rev is correct.
> We should not fall through to the " The SLRU is no longer needed." Which
> also sets the headPage to invalid. We should only truncate up to the
> head page.

Seems correct to me. Or this would count as if the SLRU is not in
use, but it's being used.
--
Michael