Re: BLK_DONE state in XLogReadBufferForRedoExtended

On Fri, Oct 13, 2017 at 10:32 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> 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.
>

I think for metapage usage, it will anyway apply the changes. See
_bt_restore_page.

> That's another case where BLK_DONE can be reached, even if
> full_page_writes is on.
>

Yeah and probably if someone uses REGBUF_NO_IMAGE. However, I was
mainly thinking about cases where caller is not doing anything to
prevent full_page_image explicitly.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com