On 5/22/2010 9:16 PM, Tom Lane wrote:
Josh Berkus <j...@agliodbs.com> writes:
Somebody (I think Joe or Heikki) poked a big hole in this last night at
the Royal Oak. Although the scheme would get rid of the need to replace
old XIDs with FrozenXid, it does not get rid of the need to set hint
bits before you can truncate CLOG. So in your example of an insert-only
table that's probably never read again, there's still a minimum of one
update visit required on every old page. Now that's still better than
two update visits ... but we could manage that already, just by tweaking
vacuum's heuristics about when to freeze vs when to set hint bits.
Yeah, someone pointed that out to me too and suggested that a freeze map
was the better solution. I still think there's something we can do with
pages on the visibility map but I'll have to think about it some more.
It occurred to me on the flight home that maybe we could salvage
something from this if there were some mechanism that caused hint bits
to get set before the page got written out from shared buffers the first
time. This assumes that you have enough slack in shared-buffer space
that the transactions that touched a particular page all commit or abort
before the page first gets flushed to disk.
At least the background writer should have a few spare cycles to look
over a "to be flushed" page before writing it.
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin
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