Re: [PERFORM] Slow Postgresql server
1= RAID 1improves data =intregrity=, not IO performance. Your HD IO performance is essentially that of 1 160GB HD of whatever performance one of those HDs have. (what kind of HDs are they anyway? For instance 7200rpm 160GB HDs are not particularly high performance) BEST case is streaming IO involving no seeks = ~50 MBps. You can't get even that as the back end of a website. 2= 1GB of RAM is -small- for a DB server. You need to buy RAM and HD. Boost the RAM to 4GB, change pg config parameters appropriately and see how much it helps. Non ECC RAM is currently running ~$60-$75 per GB for 1 or 2 GB sticks ECC RAM prices will be ~ 1.5x - 2x that, $120 - $150 per GB for 1 or 2 GB sticks. (do !not! buy 4GB sticks unless you have a large budget. Their price pr GB is still too high) If adding RAM helps as much as I suspect it will, find out how big the hot section of your DB is and see if you can buy enough RAM to make it RAM resident. If you can do this, it will result in the lowest term DB maintenance. If you can't do that for whatever reason, the next step is to improve your HD subsystem. Cheap RAID cards with enough BB cache to allow writes to be coalesced into larger streams (reducing seeks) will help, but you fundamentally you will need more HDs. RAID 5 is an reasonable option for most website DBs workloads. To hit the 300MBps speeds attainable by the cheap RAID cards, you are going to at least 7 HDs (6 HDs * 50MBps ASTR = 300MBps ASTR + the equivalent of 1 HD gets used for the R in RAID). A minimum of 8 HDs are need for this performance if you want to use RAID 6. Most tower case (not mini-tower, tower) cases can hold this internally. Price per MBps of HD is all over the map. The simplest (but not necessarily best) option is to buy more of the 160GB HDs you already have. Optimizing the money spent when buying HDs for a RAID set is a bit more complicated than doing so for RAM. Lot's of context dependent things affect the final decision. I see you are mailing from Brandeis. I'm local. Drop me some private email at the address I'm posting from if you want and I'll send you further contact info so we can talk in more detail. Cheers, Ron Peacetree At 06:02 PM 4/11/2007, Jason Lustig wrote: Hello all, My website has been having issues with our new Linux/PostgreSQL server being somewhat slow. I have done tests using Apache Benchmark and for pages that do not connect to Postgres, the speeds are much faster (334 requests/second v. 1-2 requests/second), so it seems that Postgres is what's causing the problem and not Apache. I did some reserach, and it seems that the bottleneck is in fact the hard drives! Here's an excerpt from vmstat: procs ---memory-- ---swap-- -io --system-- -cpu-- r b swpd free buff cache si sobibo incs us sy id wa st 1 1140 24780 166636 57514400 0 3900 1462 3299 1 4 49 48 0 0 1140 24780 166636 57514400 0 3828 1455 3391 0 4 48 48 0 1 1140 24780 166636 57514400 0 2440 960 2033 0 3 48 48 0 0 1140 24780 166636 57514400 0 2552 1001 2131 0 2 50 49 0 0 1140 24780 166636 57514400 0 3188 1233 2755 0 3 49 48 0 0 1140 24780 166636 57514400 0 2048 868 1812 0 2 49 49 0 0 1140 24780 166636 57514400 0 2720 1094 2386 0 3 49 49 0 As you can see, almost 50% of the CPU is waiting on I/O. This doesn't seem like it should be happening, however, since we are using a RAID 1 setup (160+160). We have 1GB ram, and have upped shared_buffers to 13000 and work_mem to 8096. What would cause the computer to only use such a small percentage of the CPU, with more than half of it waiting on I/O requests? Thanks a lot Jason ---(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 ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Slow Postgresql server
On 12.04.2007, at 07:26, Ron wrote: You need to buy RAM and HD. Before he does that, wouldn't it be more useful, to find out WHY he has so much IO? Have I missed that or has nobody suggested finding the slow queries (when you have much IO on them, they might be slow at least with a high shared memory setting). So, my first idea is, to turn on query logging for queries longer than a xy milliseconds, explain analyse these queries and see wether there are a lot of seq scans involved, which would explain the high IO. Just an idea, perhaps I missed that step in that discussion somewhere ... But yes, it might also be, that the server is swapping, that's another thing to find out. cug ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Slow Postgresql server
At 10:08 AM 4/12/2007, Guido Neitzer wrote: On 12.04.2007, at 07:26, Ron wrote: You need to buy RAM and HD. Before he does that, wouldn't it be more useful, to find out WHY he has so much IO? 1= Unless I missed something, the OP described pg being used as a backend DB for a webserver. I know the typical IO demands of that scenario better than I sometimes want to. :-( 2= 1GB of RAM + effectively 1 160GB HD = p*ss poor DB IO support. ~ 1/2 that RAM is going to be used for OS stuff, leaving only ~512MB of RAM to be used supporting pg. That RAID 1 set is effectively 1 HD head that all IO requests are going to contend for. Even if the HD in question is a 15Krpm screamer, that level of HW contention has very adverse implications. Completely agree that at some point the queries need to be examined (ditto the table schema, etc), but this system is starting off in a Bad Place for its stated purpose IME. Some minimum stuff is obvious even w/o spending time looking at anything beyond the HW config. Cheers, Ron Peacetree Have I missed that or has nobody suggested finding the slow queries (when you have much IO on them, they might be slow at least with a high shared memory setting). So, my first idea is, to turn on query logging for queries longer than a xy milliseconds, explain analyse these queries and see wether there are a lot of seq scans involved, which would explain the high IO. Just an idea, perhaps I missed that step in that discussion somewhere ... But yes, it might also be, that the server is swapping, that's another thing to find out. cug ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Slow Postgresql server
On 12.04.2007, at 08:59, Ron wrote: 1= Unless I missed something, the OP described pg being used as a backend DB for a webserver. Yep. I know the typical IO demands of that scenario better than I sometimes want to. :-( Yep. Same here. ;-) 2= 1GB of RAM + effectively 1 160GB HD = p*ss poor DB IO support. Absolutely right. Depending a little bit on the DB and WebSite layout and on the actual requirements, but yes - it's not really a kick-ass machine ... Completely agree that at some point the queries need to be examined (ditto the table schema, etc), but this system is starting off in a Bad Place for its stated purpose IME. Some minimum stuff is obvious even w/o spending time looking at anything beyond the HW config. Depends. As I said - if the whole DB fits into the remaining space, and a lot of website backend DBs do, it might just work out. But this seems not to be the case - either the site is chewing on seq scans all the time which will cause I/O or it is bound by the lack of memory and swaps the whole time ... He has to find out. cug ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Slow Postgresql server
On Thu, 12 Apr 2007, Jason Lustig wrote: 0 -- BM starts here 10 0180 700436 16420 9174000 0 176 278 2923 59 41 0 0 0 11 0180 696736 16420 9174000 0 0 254 2904 57 43 0 0 0 12 0180 691272 16420 9174000 0 0 255 3043 60 39 1 0 0 9 0180 690396 16420 9174000 0 0 254 3078 63 36 2 0 0 Obviously, I've turned off logging now but I'd like to get it running again (without bogging down the server) so that I can profile the system and find out which queries I need to optimize. My logging settings (with unnecessary comments taken out) were: So what did you get in the logs when you had logging turned on? If you have the statement logging, perhaps it's worth running through pgfouine to generate a report. log_destination = 'syslog'# Valid values are combinations of redirect_stderr = off # Enable capturing of stderr into log log_min_duration_statement = 0 # -1 is disabled, 0 logs all statements silent_mode = on# DO NOT USE without syslog or log_duration = off log_line_prefix = 'user=%u,db=%d' # Special values: log_statement = 'none' # none, ddl, mod, all Perhaps you just want to log slow queries 100ms? But since you don't seem to know what queries you're running on each web page, I'd suggest you just turn on the following and run your benchmark against it, then turn it back off: log_duration = on log_statement = 'all' Then go grab pgfouine and run the report against the logs to see what queries are chewing up all your time. So you know, we're using Postgres 8.2.3. The database currently is pretty small (we're just running a testing database right now with a few megabytes of data). No doubt some of our queries are slow, but I was concerned because no matter how slow the queries were (at most the worst were taking a couple of msecs anyway), I was getting ridiculously slow responses from the server. Outside of logging, our only other non-default postgresql.conf items are: shared_buffers = 13000 # min 128kB or max_connections*16kB work_mem = 8096 # min 64kB In terms of the server itself, I think that it uses software raid. How can I tell? Our hosting company set it up with the server so I guess I could ask them, but is there a program I can run which will tell me the information? I also ran bonnie++ and got this output: Version 1.03 --Sequential Output-- --Sequential Input- --Random- -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks-- MachineSize K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP pgtest 2000M 29277 67 33819 15 15446 4 35144 62 48887 5 152.7 0 --Sequential Create-- Random Create -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete-- files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP 16 17886 77 + +++ + +++ 23258 99 + +++ + +++ So I'm getting 33MB and 48MB write/read respectively. Is this slow? Is there anything I should be doing to optimize our RAID configuration? It's not fast, but at least it's about the same speed as an average IDE drive from this era. More disks would help, but since you indicate the DB fits in RAM with plenty of room to spare, how about you update your effective_cache_size to something reasonable. You can use the output of the 'free' command and take the cache number and divide by 8 to get a reasonable value on linux. Then turn on logging and run your benchmark. After that, run a pgfouine report against the log and post us the explain analyze from your slow queries. And if Ron is indeed local, it might be worthwhile to contact him. Someone onsite would likely get this taken care of much faster than we can on the mailing list. -- Jeff Frost, Owner [EMAIL PROTECTED] Frost Consulting, LLC http://www.frostconsultingllc.com/ Phone: 650-780-7908 FAX: 650-649-1954 ---(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] Slow Postgresql server
On Thu, 2007-04-12 at 10:19, Guido Neitzer wrote: On 12.04.2007, at 08:59, Ron wrote: Depends. As I said - if the whole DB fits into the remaining space, and a lot of website backend DBs do, it might just work out. But this seems not to be the case - either the site is chewing on seq scans all the time which will cause I/O or it is bound by the lack of memory and swaps the whole time ... He has to find out. It could also be something as simple as a very bloated data store. I'd ask the user what vacuum verbose says at the end ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Slow Postgresql server
On Thu, 12 Apr 2007, Scott Marlowe wrote: On Thu, 2007-04-12 at 10:19, Guido Neitzer wrote: On 12.04.2007, at 08:59, Ron wrote: Depends. As I said - if the whole DB fits into the remaining space, and a lot of website backend DBs do, it might just work out. But this seems not to be the case - either the site is chewing on seq scans all the time which will cause I/O or it is bound by the lack of memory and swaps the whole time ... He has to find out. It could also be something as simple as a very bloated data store. I'd ask the user what vacuum verbose says at the end You know, I should answer emails at night...we didn't ask when the last time the data was vacuumed or analyzed and I believe he indicated that the only non-default values were memory related, so no autovacuum running. Jason, Before you go any further, run 'vacuum analyze;' on your DB if you're not doing this with regularity and strongly consider enabling autovacuum. -- Jeff Frost, Owner [EMAIL PROTECTED] Frost Consulting, LLC http://www.frostconsultingllc.com/ Phone: 650-780-7908 FAX: 650-649-1954 ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [PERFORM] Slow Postgresql server
Jeff Frost wrote: You know, I should answer emails at night... Indeed you shouldN'T ;-) Carlos -- ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Slow Postgresql server
Hi all, Wow! That's a lot to respond to. Let me go through some of the ideas... First, I just turned on autovacuum, I forgot to do that. I'm not seeing a major impact though. Also, I know that it's not optimal for a dedicated server. It's not just for postgres, it's also got our apache server on it. We're just getting started and didn't want to make the major investment right now in getting the most expensive server we can get. Within the next year, as our traffic grows, we will most likely upgrade, but for now when we're in the beginning phases of our project, we're going to work with this server. In terms of RAID not helping speed-wise (only making an impact in data integrity) - I was under the impression that even a mirrored disk set improves speed, because read requests can be sent to either of the disk controllers. Is this incorrect? I turned on logging again, only logging queries 5ms. and it caused the same problems. I think it might be an issue within the OS's logging facilities, since it's going through stderr. Some of the queries are definitely making an impact on the speed. We are constantly trying to improve performance, and part of that is reassessing our indexes and denormalizing data where it would help. We're also doing work with memcached to cache the results of some of the more expensive operations. Thanks for all your help guys - it's really fantastic to see the community here! I've got a lot of database experience (mostly with ms sql and mysql) but this is my first time doing serious work with postgres and it's really a great system with great people too. Jason On Apr 12, 2007, at 11:35 AM, Jeff Frost wrote: On Thu, 12 Apr 2007, Jason Lustig wrote: 0 -- BM starts here 10 0180 700436 16420 9174000 0 176 278 2923 59 41 0 0 0 11 0180 696736 16420 9174000 0 0 254 2904 57 43 0 0 0 12 0180 691272 16420 9174000 0 0 255 3043 60 39 1 0 0 9 0180 690396 16420 9174000 0 0 254 3078 63 36 2 0 0 Obviously, I've turned off logging now but I'd like to get it running again (without bogging down the server) so that I can profile the system and find out which queries I need to optimize. My logging settings (with unnecessary comments taken out) were: So what did you get in the logs when you had logging turned on? If you have the statement logging, perhaps it's worth running through pgfouine to generate a report. log_destination = 'syslog'# Valid values are combinations of redirect_stderr = off # Enable capturing of stderr into log log_min_duration_statement = 0 # -1 is disabled, 0 logs all statements silent_mode = on# DO NOT USE without syslog or log_duration = off log_line_prefix = 'user=%u,db=%d' # Special values: log_statement = 'none' # none, ddl, mod, all Perhaps you just want to log slow queries 100ms? But since you don't seem to know what queries you're running on each web page, I'd suggest you just turn on the following and run your benchmark against it, then turn it back off: log_duration = on log_statement = 'all' Then go grab pgfouine and run the report against the logs to see what queries are chewing up all your time. So you know, we're using Postgres 8.2.3. The database currently is pretty small (we're just running a testing database right now with a few megabytes of data). No doubt some of our queries are slow, but I was concerned because no matter how slow the queries were (at most the worst were taking a couple of msecs anyway), I was getting ridiculously slow responses from the server. Outside of logging, our only other non-default postgresql.conf items are: shared_buffers = 13000 # min 128kB or max_connections*16kB work_mem = 8096 # min 64kB In terms of the server itself, I think that it uses software raid. How can I tell? Our hosting company set it up with the server so I guess I could ask them, but is there a program I can run which will tell me the information? I also ran bonnie++ and got this output: Version 1.03 --Sequential Output-- --Sequential Input- --Random- -Per Chr- --Block-- -Rewrite- -Per Chr- -- Block-- --Seeks-- MachineSize K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec % CP /sec %CP pgtest 2000M 29277 67 33819 15 15446 4 35144 62 48887 5 152.7 0 --Sequential Create-- Random Create -Create-- --Read--- -Delete-- -Create-- -- Read--- -Delete-- files /sec %CP /sec %CP /sec %CP /sec %CP /sec % CP /sec %CP 16 17886 77 + +++ + +++ 23258 99 + ++ + + +++ So I'm getting 33MB and 48MB write/read respectively. Is this
Re: [PERFORM] Slow Postgresql server
On 12.04.2007, at 15:58, Jason Lustig wrote: Wow! That's a lot to respond to. Let me go through some of the ideas... First, I just turned on autovacuum, I forgot to do that. I'm not seeing a major impact though. Also, I know that it's not optimal for a dedicated server. Hmm, why not? Have you recently vacuumed your db manually so it gets cleaned up? Even a vacuum full might be useful if the db is really bloated. It's not just for postgres, it's also got our apache server on it. We're just getting started and didn't want to make the major investment right now in getting the most expensive server we can get Hmmm, but more RAM would definitely make sense, especially in that szenaria. It really sounds like you machine is swapping to dead. What does the system say about memory usage? Some of the queries are definitely making an impact on the speed. We are constantly trying to improve performance, and part of that is reassessing our indexes and denormalizing data where it would help. We're also doing work with memcached to cache the results of some of the more expensive operations. Hmmm, that kills you even more, as it uses RAM. I really don't think at the moment that it has something to do with PG itself, but with not enough memory for what you want to achieve. What perhaps helps might be connection pooling, so that not so many processes are created for the requests. It depends on your middle- ware what you can do about that. pg_pool might be an option. cug ---(end of broadcast)--- TIP 6: explain analyze is your friend
[PERFORM] Slow Postgresql server
Hello all, My website has been having issues with our new Linux/PostgreSQL server being somewhat slow. I have done tests using Apache Benchmark and for pages that do not connect to Postgres, the speeds are much faster (334 requests/second v. 1-2 requests/second), so it seems that Postgres is what's causing the problem and not Apache. I did some reserach, and it seems that the bottleneck is in fact the hard drives! Here's an excerpt from vmstat: procs ---memory-- ---swap-- -io --system-- -cpu-- r b swpd free buff cache si sobibo incs us sy id wa st 1 1140 24780 166636 57514400 0 3900 1462 3299 1 4 49 48 0 0 1140 24780 166636 57514400 0 3828 1455 3391 0 4 48 48 0 1 1140 24780 166636 57514400 0 2440 960 2033 0 3 48 48 0 0 1140 24780 166636 57514400 0 2552 1001 2131 0 2 50 49 0 0 1140 24780 166636 57514400 0 3188 1233 2755 0 3 49 48 0 0 1140 24780 166636 57514400 0 2048 868 1812 0 2 49 49 0 0 1140 24780 166636 57514400 0 2720 1094 2386 0 3 49 49 0 As you can see, almost 50% of the CPU is waiting on I/O. This doesn't seem like it should be happening, however, since we are using a RAID 1 setup (160+160). We have 1GB ram, and have upped shared_buffers to 13000 and work_mem to 8096. What would cause the computer to only use such a small percentage of the CPU, with more than half of it waiting on I/O requests? Thanks a lot Jason ---(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] Slow Postgresql server
Jason Lustig skrev: and work_mem to 8096. What would cause the computer to only use such a small percentage of the CPU, with more than half of it waiting on I/O requests? Do your webpages write things to the database on each connect? Maybe it do a bunch of writes each individually commited? For every commit pg will wait for the data to be written down to the disk platter before it move on. So if you do several writes you want to do them in one transaction so you only need one commit. /Dennis ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [PERFORM] Slow Postgresql server
On Wed, 11 Apr 2007, Jason Lustig wrote: Hello all, My website has been having issues with our new Linux/PostgreSQL server being somewhat slow. I have done tests using Apache Benchmark and for pages that do not connect to Postgres, the speeds are much faster (334 requests/second v. 1-2 requests/second), so it seems that Postgres is what's causing the problem and not Apache. I did some reserach, and it seems that the bottleneck is in fact the hard drives! Here's an excerpt from vmstat: procs ---memory-- ---swap-- -io --system-- -cpu-- r b swpd free buff cache si sobibo incs us sy id wa st 1 1140 24780 166636 57514400 0 3900 1462 3299 1 4 49 48 0 0 1140 24780 166636 57514400 0 3828 1455 3391 0 4 48 48 0 1 1140 24780 166636 57514400 0 2440 960 2033 0 3 48 48 0 0 1140 24780 166636 57514400 0 2552 1001 2131 0 2 50 49 0 0 1140 24780 166636 57514400 0 3188 1233 2755 0 3 49 48 0 0 1140 24780 166636 57514400 0 2048 868 1812 0 2 49 49 0 0 1140 24780 166636 57514400 0 2720 1094 2386 0 3 49 49 0 As you can see, almost 50% of the CPU is waiting on I/O. This doesn't seem like it should be happening, however, since we are using a RAID 1 setup (160+160). We have 1GB ram, and have upped shared_buffers to 13000 and work_mem to 8096. What would cause the computer to only use such a small percentage of the CPU, with more than half of it waiting on I/O requests? Well, the simple answer is a slow disk subsystem. Is it hardware or software RAID1? If hardware, what's the RAID controller? Based on your vmstat output, I'd guess that this query activity is all writes since I see only blocks out. Can you identify what the slow queries are? What version of postgres? How large is the database? Can you post the non-default values in your postgresql.conf? I'd suggest you test your disk subsystem to see if it's as performant as you think with bonnie++. Here's some output from my RAID1 test server: Version 1.03 --Sequential Output-- --Sequential Input- --Random- -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks-- MachineSize K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP pgtest 4G 47090 92 52348 11 30954 6 41838 65 73396 8 255.9 1 --Sequential Create-- Random Create -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete-- files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP 16 894 2 + +++ 854 1 817 2 + +++ 969 2 So, that's 52MB/sec block writes and 73MB/sec block reads. That's typical of a RAID1 on 2 semi-fast SATA drives. If you're doing writes to the DB on every web page, you might consider playing with the commit_delay and commit_siblings parameters in the postgresql.conf. Also, if you're doing multiple inserts as separate transactions, you should consider batching them up in one transaction. -- Jeff Frost, Owner [EMAIL PROTECTED] Frost Consulting, LLC http://www.frostconsultingllc.com/ Phone: 650-780-7908 FAX: 650-649-1954 ---(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
