Re: Confused about a statement in WAL configs

On Mon, Jul 31, 2017 at 6:16 AM, <finzelj(at)gmail(dot)com> wrote:

> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/9.6/static/wal-configuration.html
> Description:
>
> In this section:
>
> &quot;At checkpoint time, all dirty data pages are flushed to disk and a
> special
> checkpoint record is written to the log file. (The change records were
> previously flushed to the WAL files.)&quot;
>
> I am confused by what the part in parenthesis means by
> &quot;previously&quot;. This
> may only reflect my ignorance, but I don&#39;t know if
> &quot;previously&quot; means &quot;in
> previous versions of Postgres&quot; or &quot;before the checkpoint&quot;
> or something
> else.
>
> Mostly because of this, I don&#39;t understand what this statement means.
> Thank
> you.
>

​"previously" in that sentence means "earlier in time". The "WAL files"
are written to during every COMMIT. They constitute a journal of changes
that affected the in-memory "pages" and made them "dirty". During a
checkpoint a marker entry is written that says a checkpoint has occurred
and that every change noted in the WAL files written before the marker
entry and now known to exist in the actual data files on disk. Until that
happens some of those change may only exist in memory (in the form of dirty
data pages).

David J.​