On 11 November 2011 23:28, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > Thom Brown <thom@xxxxxxxxx> writes: >> On 11 November 2011 00:55, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: >>> Thom Brown <thom@xxxxxxxxx> writes: >>>> I just noticed that the VACUUM process touches a lot of relations >>>> (affects mtime) but for one file I looked at, it didn't change. This >>>> doesn't always happen, and many relations aren't touched at all. > >>> No immmediate ideas as to why the mtime would change if the file >>> contents didn't. It seems like there must be a code path that marked >>> a buffer dirty without having changed it, but we're usually pretty >>> careful about that. > >> I checked all files where the time stamp of the file had changed, but >> had the same MD5 sum. I used the list in the query you mentioned and >> get: [ mostly indexes ] > > Hmm, is this on a hot standby master? It's using a wal_level of hot_standby and has max_wal_senders set to 2, but it's not actually replicating to anywhere else. But if I comment out both of these, restart, then compare pre-vacuum and post-vacuum, I get the following results for unchanged but touched items: test=# select oid,relname from pg_class where relfilenode in (11680,11682,11684,11686,11690,16530); oid | relname -------+--------------------- 2619 | pg_statistic 2840 | pg_toast_2619 2841 | pg_toast_2619_index 16530 | cows2 (4 rows) The items which didn't match a result in this instance were 11686 and 11690, which is surprising since they both have a visibility map and free space map, indicating they're some kind of table. > I observe that _bt_delitems_vacuum() unconditionally dirties the page > and writes a WAL record, whether it has anything to do or not; and that > if XLogStandbyInfoActive() then btvacuumscan will indeed call it despite > there being (probably) nothing useful to do. Seems like that could be > improved. The comment explaining why it's necessary to do that doesn't > make any sense to me, either. Well the effect, in the single instances I've checked, is certainly more pronounced for hot_standby, but there still appears to be some occurrences for minimal wal_level too. -- Thom Brown Twitter: @darkixion IRC (freenode): dark_ixion Registered Linux user: #516935 EnterpriseDB UK: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
