Re: Improve This Sentence?

On Sun, 12 Apr 2020 at 20:57, Michael Paquier <michael(at)paquier(dot)xyz> wrote:

> On Fri, Apr 10, 2020 at 06:32:05PM +0000, PG Doc comments form wrote:
> > On Linux and POSIX platforms checkpoint_flush_after allows to force the
> OS
> > that pages written by the checkpoint should be flushed to disk after a
> > configurable number of bytes.
> >
> > I think I have an idea what it says, but saw some discussion to this very
> > issue. Can it be re-written?
>
> How would you suggest to rewrite it?
>

This is interesting here is the entire paragraph in question from 29.4. WAL
Configuration

On Linux and POSIX platforms checkpoint_flush_after
<https://www.postgresql.org/docs/current/runtime-config-wal.html#GUC-CHECKPOINT-FLUSH-AFTER>
allows
to force the OS that pages written by the checkpoint should be flushed to
disk after a configurable number of bytes. Otherwise, these pages may be
kept in the OS's page cache, inducing a stall when fsync is issued at the
end of a checkpoint. This setting will often help to reduce transaction
latency, but it also can have an adverse effect on performance;
particularly for workloads that are bigger than shared_buffers
<https://www.postgresql.org/docs/current/runtime-config-resource.html#GUC-SHARED-BUFFERS>,
but smaller than the OS's page cache.

and here is another paragraph which is much better from 19.5.2. Checkpoints

Whenever more than this amount of data has been written while performing a
checkpoint, attempt to force the OS to issue these writes to the underlying
storage. Doing so will limit the amount of dirty data in the kernel's page
cache, reducing the likelihood of stalls when an fsync is issued at the end
of the checkpoint, or when the OS writes data back in larger batches in the
background. Often that will result in greatly reduced transaction latency,
but there also are some cases, especially with workloads that are bigger
than shared_buffers
<https://www.postgresql.org/docs/current/runtime-config-resource.html#GUC-SHARED-BUFFERS>,
but smaller than the OS's page cache, where performance might degrade. This
setting may have no effect on some platforms. If this value is specified
without units, it is taken as blocks, that is BLCKSZ bytes, typically 8kB.
The valid range is between 0, which disables forced writeback, and 2MB. The
default is 256kB on Linux, 0 elsewhere. (If BLCKSZ is not 8kB, the default
and maximum values scale proportionally to it.) This parameter can only be
set in the postgresql.conf file or on the server command line.

Ironically the first one above is supposed to be providing more clarity if
you can believe:
For additional information on tuning these settings, see Section 29.4.

Clearly just copying the text from 19.5.2 to 29.4 is an improvement. I'd be
somewhat loathe to be that repetitive but honestly I don't have a better
suggestion

Dave

https://www.postgres.rocks

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