Re: BLK_DONE state in XLogReadBufferForRedoExtended

On Thu, Oct 12, 2017 at 10:47 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Today, I was trying to think about cases when we can return BLK_DONE
> in XLogReadBufferForRedoExtended. One thing that occurred to me is
> that it can happen during the replay of WAL if the full_page_writes is
> off. Basically, when the full_page_writes is on, then during crash
> recovery, it will always first restore the full page image of page and
> then apply the WAL corresponding to that page, so we will never hit
> the case where the lsn of the page is greater than lsn of WAL record.
>
> Are there other cases for which we can get BLK_DONE state? Is it
> mentioned somewhere in code/comments which I am missing?

Remember the thread about meta page optimization... Some index AMs use
XLogInitBufferForRedo() to redo their meta pages and those don't have
a FPW so when doing crash recovery you may finish by not replaying a
meta page if its LSN on the page header is newer than what's being
replayed. That's another case where BLK_DONE can be reached, even if
full_page_writes is on.
--
Michael