Checkpoints questions

Hi,
Hope this helps
http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm

Thanks
DEVI.G
----- Original Message -----
From: "Henrik" <henke@mac.se>
To: <pgsql-general@postgresql.org>
Sent: Tuesday, March 04, 2008 3:28 PM
Subject: [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 gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD

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