[PERFORM] perf pb solved only after pg_dump and restore
Hi, We noticed a slowdown on our application while traffic was kinda heavy. The logics after reading the docs commanded us to trim the enlarged tables, run VACUUM ANALYZE and then expect fast performance again; but it wasn't the case[1]. Out of the blue, we dumped the database, removed it, recreated from the restore, and now the performance is lightning fast again. Does it look familiar to anyone? I thought running VACUUM ANALYZE after a trim should be enough so that pg has assembled the data and has good statistical knowledge of the tables contents.. Thanks for any tips. Ref: [1] Processes were always showing one/some postmaster on SELECT, a constant load of 1, and vmstat always showing activity in IO blocks out (application generate all sort of typical statements, some SELECT, UPDATE, INSERT either directly or through stored procedures) -- Guillaume Cottenceau Create your personal SMS or WAP Service - visit http://mobilefriends.ch/ ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] perf pb solved only after pg_dump and restore
Hi, Guillaume Guillaume Cottenceau wrote: We noticed a slowdown on our application while traffic was kinda heavy. The logics after reading the docs commanded us to trim the enlarged tables, run VACUUM ANALYZE and then expect fast performance again; but it wasn't the case[1]. What exactly do you mean with trim the enlarged tables? Out of the blue, we dumped the database, removed it, recreated from the restore, and now the performance is lightning fast again. Does it look familiar to anyone? I thought running VACUUM ANALYZE after a trim should be enough so that pg has assembled the data and has good statistical knowledge of the tables contents.. This looks like either your free_space_map setting is way to low, or you have index bloat. Maybe a VACUUM FULL fullowed by a REINDEX will have solved your problem. It also might make sense to issue a CLUSTER instead (which combines the effects of VACUUM FULL, REINDEX and physically reordering the data). 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). HTH, Markus -- Markus Schaber | Logical TrackingTracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PERFORM] perf pb solved only after pg_dump and restore
Hi Markus, Thanks for your message. Guillaume Cottenceau wrote: We noticed a slowdown on our application while traffic was kinda heavy. The logics after reading the docs commanded us to trim the enlarged tables, run VACUUM ANALYZE and then expect fast performance again; but it wasn't the case[1]. What exactly do you mean with trim the enlarged tables? 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. Out of the blue, we dumped the database, removed it, recreated from the restore, and now the performance is lightning fast again. Does it look familiar to anyone? I thought running VACUUM ANALYZE after a trim should be enough so that pg has assembled the data and has good statistical knowledge of the tables contents.. This looks like either your free_space_map setting is way to low, or you I don't know much about free_space_map. Trying to search in documentation, I found run time configuration of the two following parameters for which the current values follow: max_fsm_pages is 2 max_fsm_relations is 1000 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. Machine is a 2G Dell 1850 with lsi logic megaraid. 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.. 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? 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.. 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 :/ -- Guillaume Cottenceau Create your personal SMS or WAP Service - visit http://mobilefriends.ch/ ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PERFORM] perf pb solved only after pg_dump and restore
Guillaume, On 28 Aug 2006 11:43:16 +0200, Guillaume Cottenceau [EMAIL PROTECTED] wrote: max_fsm_pages is 2 max_fsm_relations is 1000 Do they look low? Yes they are probably too low if you don't run VACUUM on a regular basis and you have a lot of UPDATE/DELETE activity. FSM doesn't take a lot of memory so it's usually recommended to have a confortable value for it. I usually recommend to read: http://www.pervasive-postgres.com/instantkb13/article.aspx?id=10116cNode=5K1C3W http://www.pervasive-postgres.com/instantkb13/article.aspx?id=10087cNode=5K1C3W to understand better what VACUUM and FSM mean. 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.. Index slow downs write activity (you have to maintain them). It's not always a good idea to create them. 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? VACUUM ANALYZE won't help you if your database is completely bloated. And AFAICS you're not running it on a regular basis so your database was probably completely bloated which means: - bloated indexes, - bloated tables (ie a lot of fragmentation in the pages which means that you need far more pages to store the same data). The only ways to solve this situation is either to dump/restore or run a VACUUM FULL ANALYZE (VERBOSE is better to keep a log), and eventually reindex any bloated index (depends on your situation). 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 :/ You should really run VACUUM ANALYZE VERBOSE on a regular basis and analyze the logs to be sure your VACUUM strategy and FSM settings are OK. I developed http://pgfouine.projects.postgresql.org/vacuum.html to help us doing it on our production databases. Regards, -- Guillaume ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] perf pb solved only after pg_dump and restore
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 2 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 TrackingTracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] perf pb solved only after pg_dump and restore
Guillaume, Thanks for your help. On 28 Aug 2006 11:43:16 +0200, Guillaume Cottenceau [EMAIL PROTECTED] wrote: max_fsm_pages is 2 max_fsm_relations is 1000 Do they look low? Yes they are probably too low if you don't run VACUUM on a regular basis and you have a lot of UPDATE/DELETE activity. FSM doesn't take a lot of memory so it's usually recommended to have a confortable value for it. Normally, we run VACUUM ANALYZE overnight. I'd say we have low DELETE activity, kinda high SELECT/INSERT activity, and UPDATE would be in the middle of that. I usually recommend to read: http://www.pervasive-postgres.com/instantkb13/article.aspx?id=10116cNode=5K1C3W http://www.pervasive-postgres.com/instantkb13/article.aspx?id=10087cNode=5K1C3W to understand better what VACUUM and FSM mean. Thanks for the pointer, will read that. 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.. Index slow downs write activity (you have to maintain them). It's not always a good idea to create them. Of course. How newbie did I look :/. The thing is that I once did a few measurements and noticed no (measurable) impact in INSERT with a supplementary index, so I (wrongly) forgot about this. 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? VACUUM ANALYZE won't help you if your database is completely bloated. What do you mean exactly by bloated? If you mean that there is a lot of (unused) data, the thing is that our trim removed most of it. I was kinda hoping that after analyzing the database, the old data would exit the whole picture, which obviously wasn't the case. About REINDEX: is it ok to consider that REINDEX is to indexes what VACUUM FULL is to table data, because it cleans up unused index pages? And AFAICS you're not running it on a regular basis so your database was probably completely bloated which means: - bloated indexes, - bloated tables (ie a lot of fragmentation in the pages which means that you need far more pages to store the same data). I suppose that table fragmentation occurs when DELETE are interleaved with INSERT? The only ways to solve this situation is either to dump/restore or run a VACUUM FULL ANALYZE (VERBOSE is better to keep a log), and eventually reindex any bloated index (depends on your situation). Ok. 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 :/ You should really run VACUUM ANALYZE VERBOSE on a regular basis and analyze the logs to be sure your VACUUM strategy and FSM settings are OK. VACUUM ANALYZE is normally run overnight (each night). Is it not regular enough? There can be hundreds of thousands of statements a day. -- Guillaume Cottenceau Create your personal SMS or WAP Service - visit http://mobilefriends.ch/ ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] perf pb solved only after pg_dump and restore
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. Yes. So I assume that you never UPDATE, but only INSERT new entries and sometimes DELETE a big bunch of entries from the beginning. Actually, in the version of software where we have the problem, that's exactly the case. But in newer versions, UPDATE come into the picture (typically on recently inserted rows - one or two updates per row). Does UPDATE change anything? Row selection is done on the primary key (of SERIAL type). 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. Yes, that what I had in mind. But I assumed that performance would be reclaimed (as if VACUUM FULL was run) because the statistics after analyzing are accurate as to data distribution, only disk space would not be reclaimed (but we don't care, at least for the moment). 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. I actually assumed that VACUUM ANALYZE would order rows sequentially on disk (mainly because it was taking quite some time and a lot of disk output activity), but obviously this was wrong. So, I think, in your case VACUUM FULL and CLUSTER would both have solved your problem. Ok. max_fsm_pages is 2 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. max_fsm_pages = 675000 means we also need to enlarge shared buffers, or the shared buffers available space for data caching would be reduced, right? I guess the bottom line is that I don't understand what the Free Space Map behaviour really is. Is it a map containing location of free disk pages, free meaning that they correspond to pages removed with DELETE but not yet released to the OS with VACUUM FULL, which are used for INSERT in favor of enlarging the size of data used on disk? If that's correct, am I right in assuming that we don't care about the Free Space Map size if we perform a VACUUM FULL right after large bunches of DELETE? 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. We actually are obliged to use 7.4.5 :/ Am I correct in assuming that regularly running REINDEX would cut this bloat? (daily) (documentation very much insists on solving index data corruption with REINDEX and doesn't talk much about removing old obsolete data) (also, is there any way to REINDEX all index of all tables easily? as when we do just VACUUM ANALYZE for the whole database) 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. I see. However, from the additional information you gave above, I doubt it was index bloat. [...] -- Guillaume Cottenceau Create your personal SMS or WAP Service - visit http://mobilefriends.ch/ ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PERFORM] perf pb solved only after pg_dump and restore
Hi, Guillaume, Guillaume Cottenceau wrote: About REINDEX: is it ok to consider that REINDEX is to indexes what VACUUM FULL is to table data, because it cleans up unused index pages? Yes, roughly speaking. And AFAICS you're not running it on a regular basis so your database was probably completely bloated which means: - bloated indexes, - bloated tables (ie a lot of fragmentation in the pages which means that you need far more pages to store the same data). I suppose that table fragmentation occurs when DELETE are interleaved with INSERT? Yes, and it gets ugly as soon as the fsm setting is to low / VACUUM frequency is to low, so it cannot keep up. Big bunches of UPDATE/DELETE that hit more than, say 20% of the table between VACUUM runs, justify a VACUUM FULL in most cases. VACUUM ANALYZE is normally run overnight (each night). Is it not regular enough? There can be hundreds of thousands of statements a day. Which PostgreSQL version are you using? Maybe you should consider autovacuum (which is a contrib module at least since 7.4, and included in the server since 8.1). If you think that vacuum during working hours puts too much load on your server, there are options to tweak that, at least in 8.1. Markus -- Markus Schaber | Logical TrackingTracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] perf pb solved only after pg_dump and restore
Markus Schaber schabi 'at' logix-tt.com writes: VACUUM ANALYZE is normally run overnight (each night). Is it not regular enough? There can be hundreds of thousands of statements a day. Which PostgreSQL version are you using? Maybe you should consider autovacuum (which is a contrib module at least since 7.4, and included in the server since 8.1). If you think that vacuum during working hours puts too much load on your server, there are options to tweak that, at least in 8.1. Ok, thanks. Unfortunately production insists on sticking on 7.4.5 for the moment :/ -- Guillaume Cottenceau Create your personal SMS or WAP Service - visit http://mobilefriends.ch/ ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
