Re: New WAL record to detect the checkpoint redo location

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."
--
Michael