Re: AdvanceXLInsertBuffer vs. WAL segment compressibility

On 07/03/2017 06:30 PM, Chapman Flack wrote:
> On 07/03/2017 09:39 AM, Heikki Linnakangas wrote:
>
>> Hmm. That's not the problem, though. Imagine that instead of the loop
>> above, you do just:
>>
>> WALInsertLockUpdateInsertingAt(CurrPos);
>> AdvanceXLInsertBuffer(EndPos, false);
>>
>> AdvanceXLInsertBuffer() will call XLogWrite(), to flush out any pages
>> before EndPos, to make room in the wal_buffers for the new pages. Before
>> doing that, it will call WaitXLogInsertionsToFinish()
>
> Although it's moot in the straightforward approach of re-zeroing in
> the loop, it would still help my understanding of the system to know
> if there is some subtlety that would have broken what I proposed
> earlier, which was an extra flag to AdvanceXLInsertBuffer() that
> would tell it not only to skip initializing headers, but also to
> skip the WaitXLogInsertionsToFinish() check ... because I have
> the entire region reserved and I hold all the writer slots
> at that moment, it seems safe to assure AdvanceXLInsertBuffer()
> that there are no outstanding writes to wait for.

Yeah, I suppose that would work, too.

- Heikki