Re: Block-level CRC checks

Heikki Linnakangas wrote:
>> Gregory Stark wrote:
>> However you still have a problem that someone could come along and set the
>> hint bit between calculating the CRC and actually calling write.
>
> The double-buffering will solve that.

Or simply require that hint bit writes acquire a write lock on the page
(which should be available if not being critically updated or flushed);
thereby to write/flush, simply do the same, calc it's crc, write/flush to
disk, then release it's lock; and which seems like the most reliable thing
to do (although no expert on pg's implementation by any means).

As I would guess although there may be occasional lock contention, its
likely minor, and greatly simplify the whole process it would seem?

(unless I misunderstand, even double buffering requires a lock, as if
multiple hint bits may be updated during the copy, the resulting copy may be
inconsistent if only partially reflecting the updates in progress)