Re: WAL consistency check facility

On Thu, Nov 3, 2016 at 12:34 PM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> On Thu, Nov 3, 2016 at 3:24 PM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
>> On Thu, Nov 3, 2016 at 2:35 AM, Michael Paquier
>>> - /* If it's a full-page image, restore it. */
>>> - if (XLogRecHasBlockImage(record, block_id))
>>> + /* If full-page image should be restored, do it. */
>>> + if (XLogRecBlockImageApply(record, block_id))
>>> Hm. It seems to me that this modification is incorrect. If the page
>>> does not need to be applied, aka if it needs to be used for
>>> consistency checks, what should be done is more something like the
>>> following in XLogReadBufferForRedoExtended:
>>> if (Apply(record, block_id))
>>> return;
>>> if (HasImage)
>>> {
>>> //do what needs to be done with an image
>>> }
>>> else
>>> {
>>> //do default things
>>> }
>>>
>> XLogReadBufferForRedoExtended should return a redo action
>> (block restored, done, block needs redo or block not found). So, we
>> can't just return
>> from the function if BLKIMAGE_APPLY flag is not set. It still has to
>> check whether a
>> redo is required or not.
>
> Wouldn't the definition of a new redo action make sense then? Say
> SKIPPED. None of the existing actions match the non-apply case.

As per my understanding, XLogReadBufferForRedoExtended works as follows:
1. If wal record has backup block
2. {
3. restore the backup block;
4. return BLK_RESTORED;
5. }
6. else
7. {
8. If block found in buffer
10. If lsn of block is less than last replayed record
11. return BLK_DONE;
12. else
13. return BLK_NEEDS_REDO;
14. else
15. return BLK_NOT_FOUND;
16. }
Now, we can just change step 1 as follows:
1. If wal record has backup block and it needs to be restored.

I'm not getting why we should introduce a new redo action and return
from the function beforehand.

--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com