On Wed, 2006-02-01 at 20:32, Christopher Browne wrote: > > This seems maybe a bit overkill to me. I think what would be more useful > > is if autovacuum could execute more than one vacuum at a time, and you > > could specify tables that are high priority (or possibly just say that > > all tables with less than X live tuples in them are high priority). That > > way a longer-running vacuum on a large table wouldn't prevent more > > vacuum-sensative tables (such as queues) from being vacuumed frequently > > enough. > > Actually, I can think of a case for much the opposite, namely to want > to concurrently vacuum some LARGE tables... > > Suppose you have 2 rather big tables that get updates on similar > schedules such that both will have a lot of dead tuples at similar > times. > > And suppose both of these tables are Way Large, so that they take > six hours to vacuum. > > I could argue for kicking off vacuums on both, at the same moment; > they'll both be occupying transactions for 1/4 of a day, and, with > possibly related patterns of updates, doing them one after the other > *wouldn't* forcibly get you more tuples cleaned than doing them > concurrently. > > I'm not sure that's a case to push for, either, as something > pg_autovacuum is smart enough to handle; I'm just putting out some > ideas that got enough internal discussion to suggest they were > interesting enough to let others consider... This could be a big win on databases where those two tables were on different table spaces, since vacuum now wouldn't be fighting for the same thin I/O stream twice. If the autovacuum daemon scheduled vacuums so that each tablespace had a list of vacuums to run, but then ran those sets in parallel (i.e. tablespace1 has one single vacuum running though a list while tablespace2 has its own single vacuum.) Maybe even a setting that told it the max number to run in parallel for each tablespace. After all, a tablespace running on 30 hard drives in a RAID-10 could handly several concurrent vacuums, while another tablespace running on a single drive would be well limited to one vacuum at a time.
