Re: Pre-allocating WAL files

On Sat, Jan 15, 2022 at 1:36 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Thu, Jan 6, 2022 at 3:39 AM Bossart, Nathan <bossartn@amazon.com> wrote:
> >
> > On 12/30/21, 3:52 AM, "Maxim Orlov" <orlovmg@gmail.com> wrote:
> > > I did check the patch too and found it to be ok. Check and check-world are passed.
> > > Overall idea seems to be good in my opinion, but I'm not sure where is the optimal place to put the
pre-allocation.
> > >
> > > On Thu, Dec 30, 2021 at 2:46 PM Pavel Borisov <pashkin.elfe@gmail.com> wrote:
> > >> I've checked the patch v7. It applies cleanly, code is good, check-world tests passed without problems.
> > >> I think it's ok to use checkpointer for this and the overall patch can be committed. But the seen performance
gainmakes me think again before adding this feature. I did tests myself a couple of months ago and got similar
results.
> > >> Really don't know whether is it worth the effort.
> >
> > Thank you both for your review.
>
> It may have been discussed earlier, let me ask this here - IIUC the
> whole point of pre-allocating WAL files is that creating new WAL files
> of wal_segment_size requires us to write zero-filled empty pages to
> the disk which is costly. With the advent of
> fallocate/posix_fallocate, isn't file allocation going to be much
> faster on platforms where fallocate is supported? IIRC, the
> "Asynchronous and "direct" IO support for PostgreSQL." has a way to
> use fallocate. If at all, we move ahead and use fallocate, then the
> whole point of pre-allocating WAL files becomes unnecessary?
>
> Having said above, the idea of pre-allocating WAL files is still
> relevant, given the portability of fallocate/posix_fallocate.

Adding one more point: do we have any numbers like how much total time
WAL files allocation usually takes, maybe under a high-write load
server?

Regards,
Bharath Rupireddy.