Re: incremental-checkopints

On Wed, 26 Jul 2023 at 20:58, 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 ...

You don't make sure that there's enough space for the modifications,
but does it matter from a durability point of view? As long as the
page isn't written to disk before the FPI, we can replay non-FPI (but
fsynced) WAL on top of the old version of the page that you read from
disk, instead of only trusting FPIs from WAL.

> 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?

Then you ignore the max_wal_size GUC as PostgreSQL so often already
does. At least, it doesn't do what I expect it to do at face value -
limit the size of the WAL directory to the given size.

But more reasonably, you'd keep track of the count of modified pages
that are yet to be fully WAL-logged, and keep that into account as a
debt that you have to the current WAL insert pointer when considering
checkpoint distances and max_wal_size.

---

The main issue that I see with "WAL-logging the FPI only when you
write the dirty page to disk" is that dirty page flushing also happens
with buffer eviction in ReadBuffer(). This change in behaviour would
add a WAL insertion penalty to this write, and make it a very common
occurrance that we'd have to write WAL + fsync the WAL when we have to
write the dirty page. It would thus add significant latency to the
dirty write mechanism, which is probably a unpopular change.


Kind regards,

Matthias van de Meent
Neon (https://neon.tech)