On Thu, Jul 27, 2017 at 5:48 PM, Sokolov Yura <funny.fal...@postgrespro.ru> wrote: > On 2017-07-27 11:30, Masahiko Sawada wrote: >> >> On Tue, Jul 25, 2017 at 2:27 AM, Claudio Freire <klaussfre...@gmail.com> >> wrote: >>> >>> On Mon, Jul 24, 2017 at 2:20 PM, Claudio Freire <klaussfre...@gmail.com> >>> wrote: >>>> >>>> On Mon, Jul 24, 2017 at 2:10 PM, Sokolov Yura >>>> <funny.fal...@postgrespro.ru> wrote: >>>>> >>>>> On 2017-07-24 19:11, Claudio Freire wrote: >>>>>> >>>>>> I was mostly thinking about something like the attached patch. >>>>>> >>>>>> Simple, unintrusive, and shouldn't cause any noticeable slowdown. >>>>> >>>>> >>>>> >>>>> Your change is small, clear, and currently useful for huge tables under >>>>> high update load (until "allowing vacuum to use more than 1GB memory" >>>>> is merged). >>>> >>>> >>>> In high-bloat conditions, it doesn't take long to accumulate 1GB of >>>> dead tuples (which is about 178M tuples, btw). >>>> >>>> The index scan takes way longer than the heap scan in that case. >>>> >>>>> But it still delays updating fsm until whole first batch of dead tuples >>>>> cleared (ie all indices scanned, and all heap pages cleared), and on >>>>> such >>>>> huge table it will be hours. >>>> >>>> >>>> So, true, it will get delayed considerably. But as you realized, >>>> there's not much point in trying to vacuum the FSM sooner, since it >>>> won't be accurate shortly afterwards anyway. Dead line pointers do use >>>> up a fair bit of space, especially on narrow tables. >>>> >>>> In a particular table I have that exhibits this problem, most of the >>>> time is spent scanning the index. It performs dozens of index scans >>>> before it's done, so it would vacuum the FSM quite often enough, even >>>> if I were to increase the mwm setting n-fold. >>> >>> >>> I hate to reply to myself, but I wanted to add: in any case, the case >>> I'm trying to avoid is the case where the FSM *never* gets vacuumed. >>> That's bad. But it may not be the phenomenon you're experiencing in >>> your tests. >>> >> >> I think the frequently vacuuming the FSM during long-time vacuum would >> be worth to have in order to avoid a table bloating. The patch >> triggers to vacuum the FSM after vacuumed the table and indexes but I >> think that we can have a similar mechanism for a table with no index. >> >> Regards, >> >> -- >> Masahiko Sawada >> NIPPON TELEGRAPH AND TELEPHONE CORPORATION >> NTT Open Source Software Center > > > I could be wrong, but it looks like table without index doesn't > suffer that much. Since there is no indices, there is only one stage - > scanning heap, and no quadratic behavior cause of full dead-tuple array > and repeating index vacuuming. >
The vacuuming the very large table with no index could also take a long time, and it scans and vacuums blocks one by one. So I imagined that we can vacuum the FSM once vacuumed a certain amount of blocks. And that can avoid bloating table during the long-time vacuum. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
