Re: [GENERAL] Checkpoints questions
4 mar 2008 kl. 13.45 skrev Greg Smith: On Tue, 4 Mar 2008, Henrik wrote: As a starter does anyone have some clues how to analyse this: db=# select * from pg_stat_bgwriter; checkpoints_timed | checkpoints_req | buffers_checkpoint | buffers_clean | maxwritten_clean | buffers_backend | buffers_alloc ---+-+ +---+--+-+--- 118 | 435 |1925161 | 126291 | 7 | 1397373 | 2665693 Ah, nobody has asked this question yet. This is a good sample and I'm going to assimilate it into my document that someone already suggested to you. You had 118 checkpoints that happened because of checkpoint_timeout passing. 435 of them happened before that, typically those are because checkpoint_segments was reached. This suggests you might improve your checkpoint situation by increasing checkpoint_segments, but that's not a bad ratio. Increasing that parameter and spacing checkpoints further apart helps give the checkpoint spreading logic of checkpoint_completion_target more room to work over, which reduces the average load from the checkpoint process. During those checkpoints, 1,925,161 8K buffers were written out. That means on average, a typical checkpoint is writing 3481 buffers out, which works out to be 27.2MB each. Pretty low, but that's an average; there could have been some checkpoints that wrote a lot more while others wrote nothing, and you'd need to sample this data regularly to figure that out. The background writer cleaned 126,291 buffers (cleaned=wrote out dirty ones) during that time. 7 times, it wrote the maximum number it was allowed to before meeting its other goals. That's pretty low; if it were higher, it would be obvious you could gain some improvement by increasing bgwriter_lru_maxpages. Since last reset, 2,665,693 8K buffers were allocated to hold database pages. Out of those allocations, 1,397,373 times a database backend (probably the client itself) had to write a page in order to make space for the new allocation. That's not awful, but it's not great. You might try and get a higher percentage written by the background writer in advance of when the backend needs them by increasing bgwriter_lru_maxpages, bgwriter_lru_multiplier, and decreasing bgwriter_delay--making the changes in that order is the most effective strategy. Ah, thank you Greg. I actually studied your paper before writing to this list but couldn't apply your example to mine. Now I know how I can interpret those numbers. Also thank you for the performance improvement suggestions. I think this is one of the most difficult things to understand. Knowing what parameters to tweak according to the output from pg_stat_bgwriter but you helped me a great deal. Thanks! //Henke ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [GENERAL] Checkpoints questions
On Tue, 4 Mar 2008, Henrik wrote: As a starter does anyone have some clues how to analyse this: db=# select * from pg_stat_bgwriter; checkpoints_timed | checkpoints_req | buffers_checkpoint | buffers_clean | maxwritten_clean | buffers_backend | buffers_alloc ---+-++---+--+-+--- 118 | 435 |1925161 |126291 | 7 | 1397373 | 2665693 Ah, nobody has asked this question yet. This is a good sample and I'm going to assimilate it into my document that someone already suggested to you. You had 118 checkpoints that happened because of checkpoint_timeout passing. 435 of them happened before that, typically those are because checkpoint_segments was reached. This suggests you might improve your checkpoint situation by increasing checkpoint_segments, but that's not a bad ratio. Increasing that parameter and spacing checkpoints further apart helps give the checkpoint spreading logic of checkpoint_completion_target more room to work over, which reduces the average load from the checkpoint process. During those checkpoints, 1,925,161 8K buffers were written out. That means on average, a typical checkpoint is writing 3481 buffers out, which works out to be 27.2MB each. Pretty low, but that's an average; there could have been some checkpoints that wrote a lot more while others wrote nothing, and you'd need to sample this data regularly to figure that out. The background writer cleaned 126,291 buffers (cleaned=wrote out dirty ones) during that time. 7 times, it wrote the maximum number it was allowed to before meeting its other goals. That's pretty low; if it were higher, it would be obvious you could gain some improvement by increasing bgwriter_lru_maxpages. Since last reset, 2,665,693 8K buffers were allocated to hold database pages. Out of those allocations, 1,397,373 times a database backend (probably the client itself) had to write a page in order to make space for the new allocation. That's not awful, but it's not great. You might try and get a higher percentage written by the background writer in advance of when the backend needs them by increasing bgwriter_lru_maxpages, bgwriter_lru_multiplier, and decreasing bgwriter_delay--making the changes in that order is the most effective strategy. -- * Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [GENERAL] Checkpoints questions
Hi, Hope this helps http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm Thanks DEVI.G - Original Message - From: "Henrik" <[EMAIL PROTECTED]> To: Sent: Tuesday, March 04, 2008 3:28 PM Subject: [GENERAL] Checkpoints questions Hi list, I'm using 8.3 and I've started looking at the new checkpoint features. As a starter does anyone have some clues how to analyse this: db=# select * from pg_stat_bgwriter; checkpoints_timed | checkpoints_req | buffers_checkpoint | buffers_clean | maxwritten_clean | buffers_backend | buffers_alloc ---+-+ +---+--+-+--- 118 | 435 |1925161 | 126291 |7 | 1397373 | 2665693 Thanks! //Henke ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq -- No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.21.4/1309 - Release Date: 3/3/2008 6:50 PM ---(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
