On Wed, Mar 15, 2017 at 4:50 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: > On Thu, Mar 9, 2017 at 10:21 PM, Masahiko Sawada <sawada.m...@gmail.com> > wrote: >> On Wed, Mar 8, 2017 at 1:43 AM, Peter Geoghegan <p...@bowt.ie> wrote: >>> On Sat, Mar 4, 2017 at 1:30 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: >>>>> While I can't see this explained anywhere, I'm >>>>> pretty sure that that's supposed to be impossible, which this patch >>>>> changes. >>>>> >>>> >>>> What makes you think that patch will allow pg_class.relfrozenxid to be >>>> advanced past opaque->btpo.xact which was previously not possible? >>> >>> By not reliably recycling pages in a timely manner, we won't change >>> anything about the dead page. It just sticks around. This is mostly >>> fine, but we still need VACUUM to be able to reason about it (to >>> determine if it is in fact recyclable), which is what I'm concerned >>> about breaking here. It still needs to be *possible* to recycle any >>> recyclable page at some point (e.g., when we find it convenient). >>> >>> pg_class.relfrozenxid is InvalidTransactionId for indexes because >>> indexes generally don't store XIDs. This is the one exception that I'm >>> aware of, presumably justified by the fact that it's only for >>> recyclable pages anyway, and those are currently *guaranteed* to get >>> recycled quickly. In particular, they're guaranteed to get recycled by >>> the next VACUUM. They may be recycled in the course of anti-wraparound >>> VACUUM, even if VACUUM has no garbage tuples to kill (even if we only >>> do lazy_cleanup_index() instead of lazy_vacuum_index()). This is the >>> case that this patch proposes to have us skip touching indexes for. >>> >> >> To prevent this, I think we need to not skip the lazy_cleanup_index >> when ant-wraparound vacuum (aggressive = true) even if the number of >> scanned pages is less then the threshold. This can ensure that >> pg_class.relfrozenxid is not advanced past opaque->bp.xact with >> minimal pain. Even if the btree dead page is leaved, the subsequent >> modification makes garbage on heap and then autovauum recycles that >> page while index vacuuming(lazy_index_vacuum). >> > > What about if somebody does manual vacuum and there are no garbage > tuples to clean, won't in that case also you want to avoid skipping > the lazy_cleanup_index?
Yes, in that case lazy_cleanup_index will be skipped. > Another option could be to skip updating the > relfrozenxid if we have skipped the index cleanup. This could make anti-wraparound VACUUM occurs at high frequency and we cannot skip lazy_clean_index when aggressive vacuum after all. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers