* Greg Stark <greg.stark@enterprisedb.com> [081117 03:54]:
> [sorry for top-posting - damn phone]
>
> I thought of saying that too but it doesn't really solve the problem.
> Think of what happens if someone sets a hint bit on a dirty page.
If the page is dirty from a "real change", then it has a WAL backup block
record already, so the torn-page on disk is going to be fixed with the wal
replay ... *because* of the torn-page problem already being "solved" in PG.
You don't get the hint-bits back, but that's no different from the current
state. But nobody's previously cared if hint-bits wern't set on WAL replay.
The tradeoff for CRC is:
1) Are hint-bits "worth saving"? (noting that with CRC the goal is the ability of detecting blocks that aren't
*exactly*as we wrote them)
2) Are hint-bits "nice, but not worth IO" (noting that this case can be mitigated to only pages which are hint-bit
*only*changed, not dirty with already-wal-logged changes)
--
Aidan Van Dyk Create like a god,
aidan@highrise.ca command like a king,
http://www.highrise.ca/ work like a slave.