Re: incremental-checkopints

Starting from increments checkpoint is approaching the problem from
the wrong end.

What you actually want is Atomic Disk Writes which will allow turning
off full_page_writes .

Without this you really can not do incremental checkpoints efficiently
as checkpoints are currently what is used to determine when is "the
first write to a page after checkpoint" and thereby when the full page
write is needed.




On Wed, Jul 26, 2023 at 8:58 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
>
>
> On 7/26/23 15:16, Matthias van de Meent wrote:
> > On Wed, 26 Jul 2023 at 14:41, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> >>
> >> Hello
> >>
> >> On 2023-Jul-26, Thomas wen wrote:
> >>
> >>> Hi Hackes:   I found this page :
> >>> https://pgsql-hackers.postgresql.narkive.com/cMxBwq65/incremental-checkopints,PostgreSQL
> >>> no incremental checkpoints have been implemented so far. When a
> >>> checkpoint is triggered, the performance jitter of PostgreSQL is very
> >>> noticeable. I think incremental checkpoints should be implemented as
> >>> soon as possible
> >>
> >> I think my first question is why do you think that is necessary; there
> >> are probably other tools to achieve better performance.  For example,
> >> you may want to try making checkpoint_completion_target closer to 1, and
> >> the checkpoint interval longer (both checkpoint_timeout and
> >> max_wal_size).  Also, changing shared_buffers may improve things.  You
> >> can try adding more RAM to the machine.
> >
> > Even with all those tuning options, a significant portion of a
> > checkpoint's IO (up to 50%) originates from FPIs in the WAL, which (in
> > general) will most often appear at the start of each checkpoint due to
> > each first update to a page after a checkpoint needing an FPI.
>
> Yeah, FPIs are certainly expensive and can represent huge part of the
> WAL produced. But how would incremental checkpoints make that step
> unnecessary?
>
> > If instead we WAL-logged only the pages we are about to write to disk
> > (like MySQL's double-write buffer, but in WAL instead of a separate
> > cyclical buffer file), then a checkpoint_completion_target close to 1
> > would probably solve the issue, but with "WAL-logged torn page
> > protection at first update after checkpoint" we'll probably always
> > have higher-than-average FPI load just after a new checkpoint.
> >
>
> So essentially instead of WAL-logging the FPI on the first change, we'd
> only do that later when actually writing-out the page (either during a
> checkpoint or because of memory pressure)? How would you make sure
> there's enough WAL space until the next checkpoint? I mean, FPIs are a
> huge write amplification source ...
>
> Imagine the system has max_wal_size set to 1GB, and does 1M updates
> before writing 512MB of WAL and thus triggering a checkpoint. Now it
> needs to write FPIs for 1M updates - easily 8GB of WAL, maybe more with
> indexes. What then?
>
>
> regards
>
> --
> Tomas Vondra
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
>