Re: Production block comparison facility

On 07/23/2014 05:14 PM, Michael Paquier wrote:
> On Tue, Jul 22, 2014 at 4:49 PM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
> wrote:
>
>> Then, looking at the code, we would need to tweak XLogInsert for the
>> WAL record construction to always do a FPW and to update
>> XLogCheckBufferNeedsBackup. Then for the redo part, we would need to
>> do some extra operations in the area of
>> RestoreBackupBlock/RestoreBackupBlockContents, including masking
>> operations before comparing the content of the FPW and the current
>> page.
>>
>> Does that sound right?
>>
>
> I have spent some time digging more into this idea and finished with the
> patch attached, doing the following: addition of a consistency check when
> FPW is restored and applied on a given page.
>
> The consistency check is made of two phases:
> - Apply a mask on the FPW and the current page to eliminate potential
> conflicts like hint bits for example.
> - Check that the FPW is consistent with the current page, aka the current
> page does not contain any new information that the FPW taken has not. This
> is done by checking the masked portions of the FPW and the current page.
> Also some more details:
> - If an inconsistency is found, a WARNING is simply logged.
> - The consistency check is done if current page is not empty, and if
> database has reached a consistent state.
> - The page masking API is taken from the WAL replay patch that was
> submitted in CF1 and plugged in as an independent set of API.
> - In masking stuff, to facilitate if a page is used by a sequence relation
> SEQ_MAGIC as well as the its opaque data structure are renamed and moved
> into sequence.h.
> - To facilitate debugging and comparison, the masked FPW and current page
> are also converted into hex.
> Things could be refactored and improved for sure, but this patch is already
> useful as-is so I am going to add it to the next commit fest.

I don't understand how this works. A full-page image contains the new
page contents *after* the WAL-logged operation. For example, in a heap
insert, the full-page image contains the new tuple. How can you compare
that with what's on the disk already?

ISTM you'd need to log two full-page images for every WAL record. A
before image and an after image. Then you could do a lot of checking:

1. the before image should match what's on disk already
2. the result after applying the WAL record should match the after image.

That would be more handy than the approach I used, where the page images
are logged to a separate file. You wouldn't need to deal with any new
files, as all the data is in the WAL. Verification would be done
directly in the standby, with no need to run any extra programs.

- Heikki