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POThrottling i/o in postgres's pg_dump?
 

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  1. POThrottling i/o in postgres's pg_dump?
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    <p>So we have a production database that is 32GB on a machine with 16GB of RAM. Thanks to caching this is usually not a problem at all. But whenever I start a pg_dump of the database, queries from the app servers start queueing up, and after a few minutes the queue runs away and our app grinds to a halt.</p> <p>I'll be the first to acknowledge that we have query performance issues, and we're addressing those. Meanwhile, I want to be able to run pg_dump nightly, in a way that sips from the database and doesn't take our app down. I don't care if it takes hours. Our app doesn't run any DDL, so I'm not worried about lock contention.</p> <p>Attempting to fix the problem, I'm running pg_dump with both nice and ionice. Unfortunately, this doesn't address the issue.</p> <pre><code>nice ionice -c2 -n7 pg_dump -Fc production_db -f production_db.sql </code></pre> <p>Even with ionice I still see the issue above. It appears that i/o wait and lots of seeks are causing the problem.</p> <pre><code>vmstat 1 </code></pre> <p>Shows me that iowait hovers around 20-25% and spikes to 40% sometimes. Real CPU % fluctuates between 2-5% and spikes to 70% sometimes.</p> <p>I don't believe locks are a possible culprit. When I run this query:</p> <pre><code>select pg_class.relname,pg_locks.* from pg_class,pg_locks where pg_class.relfilenode=pg_locks.relation; </code></pre> <p>I only see locks which are marked granted = 't'. We don't typically run any DDL in production -- so locks don't seem to be the issue.</p> <p>Here is output from a ps with the WCHAN column enabled:</p> <pre><code>PID WIDE S TTY TIME COMMAND 3901 sync_page D ? 00:00:50 postgres: [local] COPY 3916 - S ? 00:00:01 postgres: SELECT 3918 sync_page D ? 00:00:07 postgres: INSERT 3919 semtimedop S ? 00:00:04 postgres: SELECT 3922 - S ? 00:00:01 postgres: SELECT 3923 - S ? 00:00:01 postgres: SELECT 3924 - S ? 00:00:00 postgres: SELECT 3927 - S ? 00:00:06 postgres: SELECT 3928 - S ? 00:00:06 postgres: SELECT 3929 - S ? 00:00:00 postgres: SELECT 3930 - S ? 00:00:00 postgres: SELECT 3931 - S ? 00:00:00 postgres: SELECT 3933 - S ? 00:00:00 postgres: SELECT 3934 - S ? 00:00:02 postgres: SELECT 3935 semtimedop S ? 00:00:13 postgres: UPDATE waiting 3936 - R ? 00:00:12 postgres: SELECT 3937 - S ? 00:00:01 postgres: SELECT 3938 sync_page D ? 00:00:07 postgres: SELECT 3940 - S ? 00:00:07 postgres: SELECT 3943 semtimedop S ? 00:00:04 postgres: UPDATE waiting 3944 - S ? 00:00:05 postgres: SELECT 3948 sync_page D ? 00:00:05 postgres: SELECT 3950 sync_page D ? 00:00:03 postgres: SELECT 3952 sync_page D ? 00:00:15 postgres: SELECT 3964 log_wait_commit D ? 00:00:04 postgres: COMMIT 3965 - S ? 00:00:03 postgres: SELECT 3966 - S ? 00:00:02 postgres: SELECT 3967 sync_page D ? 00:00:01 postgres: SELECT 3970 - S ? 00:00:00 postgres: SELECT 3971 - S ? 00:00:01 postgres: SELECT 3974 sync_page D ? 00:00:00 postgres: SELECT 3975 - S ? 00:00:00 postgres: UPDATE 3977 - S ? 00:00:00 postgres: INSERT 3978 semtimedop S ? 00:00:00 postgres: UPDATE waiting 3981 semtimedop S ? 00:00:01 postgres: SELECT 3982 - S ? 00:00:00 postgres: SELECT 3983 semtimedop S ? 00:00:02 postgres: UPDATE waiting 3984 - S ? 00:00:04 postgres: SELECT 3986 sync_buffer D ? 00:00:00 postgres: SELECT 3988 - R ? 00:00:01 postgres: SELECT 3989 - S ? 00:00:00 postgres: SELECT 3990 - R ? 00:00:00 postgres: SELECT 3992 - R ? 00:00:01 postgres: SELECT 3993 sync_page D ? 00:00:01 postgres: SELECT 3994 sync_page D ? 00:00:00 postgres: SELECT </code></pre>
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    1. COIf you're on linux, see if turning on the deadline io scheduler makes a difference. We're seeing *huge* differences in response times compared to the default cfq scheduler which seems particualr bad for a db server.
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    2. COThanks nos -- I've tried switching to deadline and am also increasing readahead.
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