Re: WAL consistency check facility

On Wed, Nov 2, 2016 at 4:41 PM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
> On Wed, Nov 2, 2016 at 10:23 AM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)com> wrote:
>> On Tue, Nov 1, 2016 at 10:31 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> IMHO, your rewrite of this patch was a bit heavy-handed.
>>
>> OK... Sorry for that.
>>
>>> I haven't
>>> scrutinized the code here so maybe it was a big improvement, and if so
>>> fine, but if not it's better to collaborate with the author than to
>>> take over.
>>
>> While reviewing the code, that has finished by being a large rewrite,
>> and that was more understandable than a review looking at all the
>> small tweaks and things I have been through while reading it. I have
>> also experimented a couple of ideas with the patch that I added, so at
>> the end it proves to be a gain for everybody. I think that the last
>> patch is an improvement, if you want to make your own opinion on the
>> matter looking at the differences between both patches would be the
>> most direct way to go.
>>
> If my understanding is correct regarding this feature, last two patches
> completely break the fundamental idea of wal consistency check feature.
> I mentioned this in my last reply as well that we've to use some flag
> to indicate
> whether an image should be restored during replay or not. Otherwise,
> XLogReadBufferForRedoExtended will always restore the image skipping the usual
> redo operation. What's happening now is the following:
> 1. If wal_consistency is on, include backup block image with the wal record.
> 2. During replay, XLogReadBufferForRedoExtended always restores the backup block
> image in local buffer since XLogRecHasBlockImage is true for each block.
> 3. In checkConsistency, you compare the local buffer with the backup block image
> from the wal record. It'll always be consistent.
> This feature aims to validate whether wal replay operation is
> happening correctly or not.
> To achieve that aim, we should not alter the wal replay operation itself.

Hm... Right. That was broken. And actually, while the record-level
flag is useful so as you don't need to rely on checking
wal_consistency when doing WAL redo, the block-level flag is useful to
make a distinction between blocks that have to be replayed and the
ones that are used only for consistency, and both types could be mixed
in a record. Using it in bimg_info would be fine... Perhaps a better
name for the flag would be something like BKPIMAGE_APPLY, to mean that
the FPW needs to be applied at redo. Or BKPIMAGE_IGNORE, to bypass it
when replaying it. IS_REQUIRED_FOR_REDO is quite confusing.
--
Michael