Re: Idea for getting rid of VACUUM FREEZE on cold pages

On 5/22/2010 9:16 PM, Tom Lane wrote:
> Josh Berkus <josh@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.


Jan

-- 
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin