Re: "using previous checkpoint record at" maybe not the greatest idea?

David G. Johnston wrote:

> ​Learning by reading here...
>
> http://www.postgresql.org/docs/current/static/wal-internals.html
> """
> ​After a checkpoint has been made and the log flushed, the checkpoint's
> position is saved in the file pg_control. Therefore, at the start of
> recovery, the server first reads pg_control and then the checkpoint record;
> then it performs the REDO operation by scanning forward from the log
> position indicated in the checkpoint record. Because the entire content of
> data pages is saved in the log on the first page modification after a
> checkpoint (assuming full_page_writes is not disabled), all pages changed
> since the checkpoint will be restored to a consistent state.
>
> To deal with the case where pg_control is corrupt, we should support the
> possibility of scanning existing log segments in reverse order — newest to
> oldest — in order to find the latest checkpoint. This has not been
> implemented yet. pg_control is small enough (less than one disk page) that
> it is not subject to partial-write problems, and as of this writing there
> have been no reports of database failures due solely to the inability to
> read pg_control itself. So while it is theoretically a weak spot,
> pg_control does not seem to be a problem in practice.
> ​"""​
>
> ​The above comment appears out-of-date if this post describes what
> presently happens.

I think you're misinterpreting Andres, or the docs, or both.

What Andres says is that the control file (pg_control) stores two
checkpoint locations: the latest one, and the one before that. When
recovery occurs, it starts by looking up the latest checkpoint record;
if it cannot find that for whatever reason, it falls back to reading the
previous one. (He further claims that falling back to the previous one
is a bad idea.)

What the 2nd para in the documentation is saying is something different:
it is talking about reading all the pg_xlog files (in reverse order),
which is not pg_control, and see what checkpoint records are there, then
figure out which one to use.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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