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.
