Re: incremental-checkopints

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
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