Alex Rivera | Logout

Postgres performance issues

Asked 2012-08-13T18:55:54.123
13

We are running Postgres 9.1.3 and we have recently started to run into major performance problems on one of our servers.

Our queries ran fine for a while, but as of August 1st, they have slowed down dramatically. It would appear that most of the problematic queries are Select queries (queries with count(*) are especially bad), but in general, the database is just running really slow.

We ran this query on the server and these were the changes that we have made to the default config file (Note: The server ran fine with these changes before, so, they likely don't matter much) :

       name            |                                                current_setting
---------------------------+---------------------------------------------------------------------------------------------------------------
version                   | PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu, compiled by  gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-51), 64-bit
autovacuum                | off
bgwriter_delay            | 20ms
checkpoint_segments       | 6
checkpoint_warning        | 0
client_encoding           | UTF8
default_statistics_target | 1000
effective_cache_size      | 4778MB
effective_io_concurrency  | 2
fsync                     | off
full_page_writes          | off
lc_collate                | en_US.UTF-8
lc_ctype                  | en_US.UTF-8
listen_addresses          | *
maintenance_work_mem      | 1GB
max_connections           | 100
max_stack_depth           | 2MB
port                      | 5432
random_page_cost          | 2
server_encoding           | UTF8
shared_buffers            | 1792MB
synchronous_commit        | off
temp_buffers              | 16MB
TimeZone                  | US/Eastern
wal_buffers               | 16MB
wal_level                 | minimal
wal_writer_delay          | 10ms
work_mem                  | 16MB
(28 rows)

Time: 210.231 ms

Normall

performance postgresql
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1 Answer

11

It's difficult to be sure, but I think you are right to be suspicious of I/O issues. What can happen is that as tables get larger or connections are increased then cache hits start to fall. That increases I/o demands and slows everything down. Meanwhile, more queries arrive, making the problem worse. The situation is complicated for you because virtual disks don't necessarily behave the same as physical ones.

Firstly you will need to measure actual activity on the VM (through vmstat or iostat perhaps). Secondly, do the same on the real hardware. Finally, run some standard disk bandwidth tools on both (in particular random read/write mixes). Now you'll be able to say how much of your available I/o is being used.

As for query plans, without the schema details and explain analyse output no-one can say.

You will find the postgresql.org mailing list useful even if just for the archives. Also, the book linked below is excellent.

http://www.packtpub.com/postgresql-90-high-performance/book

answered 2012-08-13T21:27:45.533

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