On Fri, 2006-12-29 at 10:49 -0500, Tom Lane wrote: > "Simon Riggs" <[EMAIL PROTECTED]> writes: > > I would suggest that we tracked whether a block has had 0, 1 or 1+ > > updates/deletes against it. When a block has 1+ it can then be > > worthwhile to VACUUM it and to place it onto the FSM. Two dead tuples is > > really the minimum space worth reclaiming on any block. > > How do you arrive at that conclusion?
FSM code ignores any block with less space than 1 average tuple, which is a pretty reasonable rule. If you only track whether a block has been updated, not whether it has been updated twice, then you will be VACUUMing lots of blocks that have only a 50% chance of being usefully stored by the FSM. As I explained, the extra bit per block is easily regained from storing less FSM data. My understanding was that DSM was meant to increase VACUUM efficiency, so having a way to focus in on blocks most worth vacuuming makes sense using the 80/20 rule. > Counterexample: table in which all tuples exceed half a page. Current FSM code will ignore those too, if they are less than the average size of the tuple so far requested. Thats a pretty wierd counterexample, even if it is a case that needs handling. -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
