Re: New WAL record to detect the checkpoint redo location

On Wed, Aug 30, 2023 at 1:03 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Mon, Aug 28, 2023 at 01:47:18PM +0530, Dilip Kumar wrote:
> > I removed this mainly because now in other comments[1] where we are
> > introducing this new CHECKPOINT_REDO record we are explaining the
> > problem
> > that the redo location and the actual checkpoint records are not at
> > the same place and that is because of the concurrent xlog insertion.
> > I think we are explaining in more
> > detail by also stating that in case of a shutdown checkpoint, there
> > would not be any concurrent insertion so the shutdown checkpoint redo
> > will be at the same place. So I feel keeping old comments is not
> > required.
> > And we are explaining it when we are setting this for
> > non-shutdown checkpoint because for shutdown checkpoint this statement
> > is anyway not correct because for the shutdown checkpoint the
> > checkpoint record will be at the same location and there will be no
> > concurrent wal insertion, what do you think?
>
> + * Insert a dummy CHECKPOINT_REDO record and set start LSN of this record
> + * as checkpoint.redo.
>
> I would add a "for a non-shutdown checkpoint" at the end of this
> sentence.
>
> + * record. So when processing the wal, we cannot rely on the checkpoint
> + * record if we want to stop at the checkpoint-redo LSN.
>
> The term "checkpoint-redo" is also a bit confusing, I guess, because
> you just mean to refer to the "redo" LSN here? Maybe rework the last
> sentence as:
> "So, when processing WAL, we cannot rely on the checkpoint record if
> we want to stop at the same position as the redo LSN".
>
> + * This special record, however, is not required when we are doing a
> + * shutdown checkpoint, as there will be no concurrent wal insertions
> + * during that time. So, the shutdown checkpoint LSN will be the same as
> + * checkpoint-redo LSN.
>
> Perhaps the last sentence could be merged with the first one, if we
> are tweaking things, say:
> "This special record is not required when doing a shutdown checkpoint;
> the redo LSN is the same LSN as the checkpoint record as there cannot
> be any WAL activity in a shutdown sequence."

Your suggestions LGTM so modified accordingly

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com