On Mon, Mar 19, 2018 at 2:45 PM, Kyotaro HORIGUCHI <horiguchi.kyot...@lab.ntt.co.jp> wrote: > At Mon, 19 Mar 2018 11:12:58 +0900, Masahiko Sawada <sawada.m...@gmail.com> > wrote in <CAD21AoAB8tQg9xwojupUJjKD=fmhtx6thdependdhftvlwc...@mail.gmail.com> >> On Wed, Mar 14, 2018 at 9:25 PM, Alexander Korotkov >> <a.korot...@postgrespro.ru> wrote: >> > On Wed, Mar 14, 2018 at 7:40 AM, Masahiko Sawada <sawada.m...@gmail.com> >> > wrote: >> >> >> >> On Sat, Mar 10, 2018 at 3:40 AM, Alexander Korotkov >> >> <a.korot...@postgrespro.ru> wrote: >> >> > On Fri, Mar 9, 2018 at 3:12 PM, Masahiko Sawada <sawada.m...@gmail.com> >> >> > wrote: >> >> >> >> >> >> On Fri, Mar 9, 2018 at 8:43 AM, Alexander Korotkov >> >> >> <a.korot...@postgrespro.ru> wrote: >> >> >> > 2) These parameters are reset during btbulkdelete() and set during >> >> >> > btvacuumcleanup(). >> >> >> >> >> >> Can't we set these parameters even during btbulkdelete()? By keeping >> >> >> them up to date, we will able to avoid an unnecessary cleanup vacuums >> >> >> even after index bulk-delete. >> >> > >> >> > >> >> > We certainly can update cleanup-related parameters during >> >> > btbulkdelete(). >> >> > However, in this case we would update B-tree meta-page during each >> >> > VACUUM cycle. That may cause some overhead for non append-only >> >> > workloads. I don't think this overhead would be sensible, because in >> >> > non append-only scenarios VACUUM typically writes much more of >> >> > information. >> >> > But I would like this oriented to append-only workload patch to be >> >> > as harmless as possible for other workloads. >> >> >> >> What overhead are you referring here? I guess the overhead is only the >> >> calculating the oldest btpo.xact. And I think it would be harmless. >> > >> > >> > I meant overhead of setting last_cleanup_num_heap_tuples after every >> > btbulkdelete with wal-logging of meta-page. I bet it also would be >> > harmless, but I think that needs some testing. >> >> Agreed. >> >> After more thought, it might be too late but we can consider the >> possibility of another idea proposed by Peter. Attached patch >> addresses the original issue of index cleanups by storing the epoch >> number of page deletion XID into PageHeader->pd_prune_xid which is >> 4byte field. > > Mmm. It seems to me that the story is returning to the > beginning. Could I try retelling the story? > > I understant that the initial problem was vacuum runs apparently > unnecessary full-scan on indexes many times. The reason for that > is the fact that a cleanup scan may leave some (or many under > certain condition) dead pages not-recycled but we don't know > whether a cleanup is needed or not. They will be staying left > forever unless we run additional cleanup-scans at the appropriate > timing. > > (If I understand it correctly,) Sawada-san's latest proposal is > (fundamentally the same to the first one,) just skipping the > cleanup scan if the vacuum scan just before found that the number > of *live* tuples are increased. If there where many deletions and > insertions but no increase of total number of tuples, we don't > have a cleanup. Consequently it had a wraparound problem and it > is addressed in this version.
No, it doesn't have a wraparound problem. The patch based on Peter's idea I proposed adds an epoch number of page deletion xid and compare them when we judge whether the page is recyclable or not. It's something like we create 64-bit xid of deletion xid. Also, if there is even one deletion the bulk-delete will be performed instead of the index cleanup. So with this patch we do the index cleanup only when the reltuple of table is increased by fraction of vacuum_index_cleanup_scale_factor from previous stats. It doesn't need to do the index cleanup by any xid thresholds. > (ditto.) Alexander proposed to record the oldest xid of > recyclable pages in metapage (and the number of tuples at the > last cleanup). This prevents needless cleanup scan and surely > runs cleanups to remove all recyclable pages. Yes, but the concerns we discussed are that we need extra WAL-logging for updating the metapage and it works only for append-only case. If we also want to support more cases we will need to update the metapage during bulk-delete. The overhead of WAL-logging would be harmless but should be tested as Alexander mentioned. > > I think that we can accept Sawada-san's proposal if we accept the > fact that indexes can retain recyclable pages for a long > time. (Honestly I don't think so.) > > If (as I might have mentioned as the same upthread for Yura's > patch,) we accept to hold the information on index meta page, > Alexander's way would be preferable. The difference betwen Yura's > and Alexander's is the former runs cleanup scan if a recyclable > page is present but the latter avoids that before any recyclable > pages are knwon to be removed. > >> Comparing to the current proposed patch this patch >> doesn't need neither the page upgrade code nor extra WAL-logging. If > > # By the way, my proposal was storing the information as Yura > # proposed into stats collector. The information maybe be > # available a bit lately, but it doesn't harm. This doesn't need > # extra WAL logging nor the upgrad code:p > >> we also want to address cases other than append-only case we will > > I'm afraid that "the problem for the other cases" is a new one > that this patch introduces, not an existing one. I meant that the current Alexandor's proposal works for append-only table. If we want to support other cases we have to update metapage during bulk-delete, which assumes that bulk-delete always scans whole index. > >> require the bulk-delete method of scanning whole index and of logging >> WAL. But it leads some extra overhead. With this patch we no longer >> need to depend on the full scan on b-tree index. This might be useful >> for a future when we make the bulk-delete of b-tree index not scan >> whole index. > > Perhaps I'm taking something incorrectly, but is it just the > result of skipping 'maybe needed' scans without condiering the > actual necessity? I meant to scan only index pages that are relevant with garbages TIDs on a table. The current b-tree index bulk-deletion is very slow and heavy because we always scans the whole index even if there is only 1 dead tuples in a table. To address this problem I'm thinking a way to make bulk-delete not scan whole index if there is a few dead tuples in a table. That is, we do index scans to collect the stack of btree pages and reclaim garbage. Maybe we will full index scan if there are a lot of dead tuples, which would be same as what we're doing on planning access paths. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center
