Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

On Mon, Apr 6, 2026 at 11:32 AM Alexey Makhmutov
<a(dot)makhmutov(at)postgrespro(dot)ru> wrote:
>
> This makes sense. I'd like just to put some context here: I was checking
> the FSM update case in scope of the thread
> https://www.postgresql.org/message-id/flat/596c4f1c-f966-4512-b9c9-dd8fbcaf0928(at)postgrespro(dot)ru,
> in which I was specifically looking at the case with outdated FSM data
> (showing lots of free space) on standby causing a significant
> performance hit after switchover. As example this include case with
> table having fillfactor<=80 which has prior bulk rows deletes +
> insertion. In this case (mostly) empty FSM block may be delivered to
> standby via FPI, but subsequent inserts may be lost due to the 20%
> heuristic. Moreover, updates to FSM may be lost even for blocks filled
> for more than 80% due to missing dirty flag as described in that thread.
>
> In my understanding the FSM update during processing of the
> 'heap_xlog_visible' function on standby was kind of 'last line of
> defense' for any corner case scenario with FSM update (as block would
> not be visited by the vacuum process once it's marked as 'all visible')
> and it was introduced in
> https://www.postgresql.org/message-id/20180802172857.5skoexsilnjvgruk@alvherre.pgsql
> (ab7dbd681) specifically for this purpose. Now, as logic of
> 'heap_xlog_visible' is merged into 'heap_xlog_prune_freeze', so this
> task is carried by this function.
>
> I fully agree that having exactly zero-space seems to be a very uncommon
> situation (and probably not reproducible with tables having
> fillfactor<=80). I've just noted that such case was processed by the old
> logic in the 'heap_xlog_visible', while current implementation in
> 'heap_xlog_prune_freeze' skips it.

The scenario causing inaccurate freespace maps after promotion is
technically possible though improbable. Moreover, I don't see a
downside to changing it. Patch I plan to commit is attached.

I don't quite understand why heap_xlog_insert() considers whether the
heap page was an FPI before updating the FSM though. I know we need
some heuristic to avoid doing it for every record, but the FPI
consideration doesn't make sense to me.

- Melanie