Hi,
On 2021-05-25 22:23:46 -0400, Stephen Frost wrote:
> Andres mentioned other possible cases where the LSN doesn’t change even
> though we change the page and, as he’s probably right, we would have to
> figure out a solution in those cases too (potentially including cases like
> crash recovery or replay on a replica where we can’t really just go around
> creating dummy WAL records to get new LSNs..).
Yea, I think there's quite a few of those. For one, we don't guarantee
that that the hole between pd_lower/upper is zeroes. It e.g. contains
old tuple data after deleted tuples are pruned away. But when logging an
FPI, we omit that range. Which means that after crash recovery the area
is zeroed out. There's several cases where padding can result in the
same.
Just look at checkXLogConsistency(), heap_mask() et al for all the
differences that can occur and that need to be ignored for the recovery
consistency checking to work.
Particularly the hole issue seems trivial to exploit, because we know
the plaintext of the hole after crash recovery (0s).
I don't see how using the LSN alone is salvagable.
Greetings,
Andres Freund