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

On 5/24/2010 9:30 AM, Heikki Linnakangas wrote:
> On 22/05/10 16:35, Tom Lane wrote:
>> Josh Berkus  writes:
>>>    From a discussion at dinner at pgcon, I wanted to send this to the list
>>> for people to poke holes in it:
>>
>> Somebody (I think Joe or Heikki) poked a big hole in this last night at
>> the Royal Oak.
> 
> Me.
> 
>>  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.
> 
> Hmm, we don't rely on setting hint bits to truncate CLOG anymore 
> (http://archives.postgresql.org/pgsql-committers/2006-11/msg00026.php). 
> It's the replacement of xids with FrozenXid that matters, the hint bits 
> are really just hints.
> 
> Doesn't change the conclusion, though: you still need to replace XIDs 
> with FrozenXids to truncate the clog. Conceivably we could keep around 
> more than 2^32 transactions in clog with this scheme, but then you need 
> a lot more space for the clog. But perhaps it would be better to do that 
> than to launch anti-wraparound vacuums, or to refuse more updates in the 
> extreme cases.

Correct. The problem actually are aborted transactions. Just because an 
XID is really old doesn't mean it was committed.


Jan

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