Re: WAL consistency check facility - Mailing list pgsql-hackers

On Wed, Sep 14, 2016 at 6:34 AM, Michael Paquier
<michael.paquier@gmail.com> wrote:
>>>2) Regarding page comparison:
>>>We could just use memcpy() here. compareImages was useful to get a
>>>clear image of what the inconsistencies were, but you don't do that
>>>anymore.
>> memcmp(), right?
>
>Yep :)
If I use memcmp(), I won't get the byte location where the first mismatch
has occurred. It will be helpful to display the byte location which causes
an inconsistency.

>>>6)
>>>+           /*
>>>+            * Remember that, if WAL consistency check is enabled for
>>>the current rmid,
>>>+            * we always include backup image with the WAL record.
>>>But, during redo we
>>>+            * restore the backup block only if needs_backup is set.
>>>+            */
>>>+           if (needs_backup)
>>>+               bimg.bimg_info |= BKPIMAGE_IS_REQUIRED_FOR_REDO;
>>>This should use wal_consistency[rmid]?
>>
>> needs_backup is set when XLogRecordAssemble decides that backup image
>> should be included in the record for redo purpose. This image will be
>> restored during
>> redo. BKPIMAGE_IS_REQUIRED_FOR_REDO indicates whether the included
>> image should be restored during redo(or has_image should be set or not).
>
>When decoding a record, I think that you had better not use has_image
>to assume that a FPW has to be double-checked. This has better be a
>different boolean flag, say check_page or similar. This way after
>decoding a record it is possible to know if there is a PFW, and if a
>check on it is needed or not.
I've done some modifications which discards the necessity of adding
anything in DecodeXLogRecord().

Master
---------------
- If wal_consistency check is enabled or needs_backup is set in
XLogRecordAssemble(), we do a fpw.
- If a fpw is to be done, then fork_flags is set with BKPBLOCK_HAS_IMAGE,
which in turns set has_image flag while decoding the record.
- If a fpw needs to be restored during redo, i.e., needs_backup is true,
then bimg_info is set with BKPIMAGE_IS_REQUIRED_FOR_REDO.

Standby
---------------
- In XLogReadBufferForRedoExtended(), if both XLogRecHasBlockImage() and
XLogRecHasBlockImageForRedo()(added by me*) return true, we restore the
backup image.
- In checkConsistency, we only check if XLogRecHasBlockImage() returns true
when wal_consistency check is enabled for this rmid.

*XLogRecHasBlockImageForRedo() checks whether bimg_info is set with
BKPIMAGE_IS_REQUIRED_FOR_REDO.

Thoughts?

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