Hi, Guillaume, Guillaume Cottenceau wrote: > We have a couple of logs files which get larger over time > (millions of rows). As they are log files, they can be trimmed > from older values. Ah, ok, you DELETEd the old rows. So I assume that you never UPDATE, but only INSERT new entries and sometimes DELETE a big bunch of entries from the beginning. This is a special usage pattern, where the normal "VACUUM" is not well suited for. DELETing rows itsself does not free any space. Only after your transaction is committed, a following VACUUM FULL or CLUSTER does actually free the space. VACUUM and VACUUM ANALYZE only remove obsolete rows from the pages and marks them free (by entering them into the free space map, as long as that one is large enough). That means that your table will actually stay as large as before, having 90% of free pages at the beginning and 10% used pages at the very end. New INSERTs and UPDATEs will prefer to use pages from the free space map before allocating new pages, but the existing rows will stay forever. Now, VACUUM FULL actively moves rows to the beginning of the table, allowing to cut the end of the table, while CLUSTER recreates the table from scratch, in index order. Both lead to a compact storage, having all used rows at the beginning, and no free pages. So, I think, in your case VACUUM FULL and CLUSTER would both have solved your problem. > max_fsm_pages is 20000 > Do they look low? > Notice: table data is only 600M after trim (without indexes), > while it was probably 3x to 10x this size before the trim. 10x the size means 6G, so 5.4G of data were freed by the trim. Each page has 8k in size, so the fsm needs about 675000 pages. So, yes, for your usage, they look low, and give very suboptimal results. >> have index bloat. > > Can you elaborate? I have created a couple of indexes (according > to multiple models of use in our application) and they do take up > quite some disk space (table dump is 600M but after restore it > takes up 1.5G on disk) but I thought they could only do good or > never be used, not impair performance.. Like tables, indices may suffer from getting bloated by old, unused entries. Especially the GIST based indices in 7.4 (used by PostGIS and other plugins) suffered from that problem[1], but recent PostgreSQL versions have improved in this area. Now, when the query planner decides to use an index, the index access is extremely slow because of all the deleted entries the index scan has to skip. However, from the additional information you gave above, I doubt it was index bloat. >> Maybe a VACUUM FULL fullowed by a REINDEX will have solved your problem. > > So these would have reordered the data for faster sequential > access which is not the case of VACUUM ANALYZE? A VACUUM FULL would have reordered the data, and a REINDEX would have optimized the index. >> It also might make sense to issue a CLUSTER instead (which combines the >> effects of VACUUM FULL, REINDEX and physically reordering the data). > > I was reluctant in using CLUSTER because you have to choose an > index and there are multiple indexes on the large tables.. Usually, CLUSTERing on one index does not necessarily slow down accesses on other indices, compared to the non-clustered (= random) table before. If you have some indices that are somehow related (e. G. a timestamp and a serial number), CLUSTERing on one index does automatically help the other index, especially as the query planer uses corellation statistics. Btw, if your queries often include 2 or 3 columns, a multi-column index (and clustering on that index) might be the best. >> When the free_space_map is to low, VACUUM ANALYZE should have told you >> via a warning (at least, if your logging is set appropriately). > > Unfortunately, we didn't keep the logs of VACUUM ANALYZE, so I > can't be sure :/ AFAIK, the warning is also output on the psql command line. HTH, Markus [1] We once had an index that was about 100 times larger before REINDEX. -- Markus Schaber | Logical Tracking&Tracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org
