Re: WAL consistency check facility

On Wed, Sep 14, 2016 at 11:31 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> On Wed, Sep 14, 2016 at 2:56 PM, Kuntal Ghosh
> <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
>> 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.
>
> Here that should be if wal_consistency is true, no?
Nope. I'll try to explain using some pseudo-code:
XLogRecordAssemble()
{
....
include_image = needs_backup || wal_consistency[rmid];
if (include_image)
{
....
set XLogRecordBlockHeader.fork_flags |= BKPBLOCK_HAS_IMAGE;
if (needs_backup)
set XLogRecordBlockImageHeader.bimg_info
|= BKPIMAGE_IS_REQUIRED_FOR_REDO;
....
}
.....
}

XLogReadBufferForRedoExtended()
{
......
if (XLogRecHasBlockImage() && XLogRecHasBlockImageForRedo())
{
RestoreBlockImage();
....
return BLK_RESTORED;
}
......
}

checkConsistency()
{
....
if (wal_consistency[rmid] && !XLogRecHasBlockImage())
throw error;
.....
}

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

For a backup image any of the followings is possible:
1. consistency should be checked.
2. page should restored.
3. both 1 and 2.

Consistency check can be controlled by a guc parameter. But, standby
should be conveyed whether an image should be restored. For that, we
have used the new flag.
Suggestions?

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