Re: max_db_connections

On 31-10-14 15:28, Rogerio Pereira wrote:
> Hi,
> 
> I need suggestions about max_db_connections.
> 
> Imapd usually hangs in my DBmail instance running 3.1.17 in a daily basis.
> 
> /dbmail-imapd[10702]: [0x18e6f90] Alert:[db] db_con_get(+254): Thread is
> having trouble obtaining a database connection. Try [0]/
> 
> / pool[10726]: segfault at 0 ip:00007fc0c2b53e20 sp 00007fc0b9fa5660
> error 4 in libzdb.so.9.0.2[7fc0c2b4b000+1a/
> 
> I decided to change max_db_connections from 10 to 20.
> 
> Is it OK? What are the chances to get it worst than before? My server
> has 1900 users, average of 30 simultaneous user connections, PgSQL 9.3
> in a separate server and 120 GB total DB size.

Ok. Nice. The Alert in itself is not a problem. DBMail will retry to
obtain a connection after waiting for 1 second, and retry until it can
get one.

If you see ... Try [x] where x keeps growing, then you might want to
consider increasing the max_db_connections. But if the database is
already very busy that will not help very much, unless your database
server is a really big iron.

> 
> Another question: Pop3d does not throw this error in dbmail.err. I guess
> this service will be impacted by the same DB imapd connection issue. Is
> this pop3d error a symptom of the same problem?

No. pop3d is event-driven but single threaded, and will never handle
more than one single database connection concurrently.

>  dbmail-pop3d[17231] general protection ip:7ffd411f180a sp:7fff0c6799c8
> error:0 in libdbmail.so.0.0.0[7ffd411ad000+6b000]

With that many users I would start load-balancing:

 [nginx] -> [dbmail-1...x] -> [db]

where nginx (or another mail proxy like dovecot or haproxy) will
distribute connections over a number of dbmail instances.

Also allows for better uptime during restarts, etc.

-- 
________________________________________________________________
Paul J Stevens       pjstevns @ gmail, twitter, github, linkedin
           www.nfg.nl/[email protected]/+31.85.877.99.97