Re: Protecting against unexpected zero-pages: proposal

I wrote:
> Aidan Van Dyk <aidan(at)highrise(dot)ca> writes:
>> Getting back to the checksum debate (and this seems like a
>> semi-version of the checksum debate), now that we have forks, could we
>> easily add block checksumming to a fork?

> More generally, this re-opens the question of whether data in secondary
> forks is authoritative or just hints. Currently, we treat it as just
> hints, for both FSM and VM, and thus sidestep the problem of
> guaranteeing its correctness. To use a secondary fork for checksums,
> you'd need to guarantee correctness of writes to it.

... but wait a minute. What if we treated the checksum as a hint ---
namely, on checksum failure, we just log a warning rather than doing
anything drastic? A warning is probably all you want to happen anyway.

A corrupted page of checksums would then show up as warnings for most or
all of a range of data pages, and it'd be pretty obvious (if the data
seemed OK) where the failure had been.

So maybe Aidan's got a good idea here. It would sure be a lot easier
to shoehorn checksum checking in as an optional feature if the checksums
were kept someplace else.

regards, tom lane