mysql connection through socket fails

Question

Situation is that, we have AWS EC2 medium instance, with Linux on it.
It has Drupal as well. Additionally to that we have few files that access to mysql as well, with same settings as Drupal does.
The problem is that - at one point mysql refuses connections.
It happens when there is low or big load (not related on this), as well once it gets inaccessible, mysqld process is still running, and does not falls.
Restarting of this process does not fixes a problem. Rebooting instance - fixes problem.

When i connect to localhost it gives this:

Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)

While mysql.sock file is on place and with right permissions.
Restarting mysqld doesn't helps, but restarting instance - solves the problem.

my.cnf looks like that:

[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
symbolic-links=0

wait_timeout=28800

interactive_timeout = 28800

max_allowed_packet=32M

[mysqld_safe]
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid

As well mysqld runs without any errors, and in logs we have this:

120830  9:48:00 [Note] /usr/libexec/mysqld: Shutdown complete

120830 09:48:00 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
120830 09:48:01 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
120830  9:48:01 [Note] Plugin 'FEDERATED' is disabled.
120830  9:48:01 InnoDB: The InnoDB memory heap is disabled
120830  9:48:01 InnoDB: Mutexes and rw_locks use GCC atomic builtins
120830  9:48:01 InnoDB: Compressed tables use zlib 1.2.3
120830  9:48:01 InnoDB: Using Linux native AIO
120830  9:48:01 InnoDB: Initializing buffer pool, size = 128.0M
120830  9:48:01 InnoDB: Completed initialization of buffer pool
120830  9:48:02 InnoDB: highest supported file format is Barracuda.
120830  9:48:02  InnoDB: Waiting for the background threads to start
120830  9:48:03 InnoDB: 1.1.8 started; log sequence number 4191070086
120830  9:48:03 [Note] Event Scheduler: Loaded 0 events
120830  9:48:03 [Note] /usr/libexec/mysqld: ready for connections.
Version: '5.5.20'  socket: '/var/lib/mysql/mysql.sock -u root'  port: 3306  MySQL Community Server (GPL)

When issue occured again, I took logs again, tried to stop httpd then mysqld, then run mysqld and then run httpd, and logs are absolutely the same as in normal conditions logs with same restart sequence.

Changing in php.ini did not saved the situation:

mysql.allow_persistent = Off

Restarting in this order, does not helps (even different orders tried):

service httpd stop
service mysqld stop
service mysqld start
service httpd start

We would like to find out what is problem and how prevent it from falling like that.

Answer 1

I am not a very experienced user in this subject but when I had some socket files trouble, I configured my apps to use TCP/IP instead. You may use 127.0.0.1 instead of localhost in your software configuration, to force TCP/IP instead of socket file.

You might be interested in the Xiaofeng Teng answer to another stackoverflow question :

Besides Michael's words,

there's another link: http://dev.mysql.com/doc/refman/5.1/en/connecting.html, it says:

On Unix, MySQL programs treat the host name localhost specially, in a way that is likely different from what you expect compared to other network-based programs. For connections to localhost, MySQL programs attempt to connect to the local server by using a Unix socket file. This occurs even if a --port or -P option is given to specify a port number.

it's not a typical tcp/ip connection.

This, of course, will not answer your question, but perhaps solve your problem.

Answer 2

Just glancing over your configuration: Your timeouts are insanely high. As others have guessed, I think you're trying to use persistent connections. But these usually never work with standard ext/mysql(i) or ext/PDO`.

If you don't want to toy with something like the the new mysqlnd multiplexing plugin (see intro, see FAQ, I would suggest to lower wait_timeout significantly and also watch max_connections (on the mysqld-side) when you experience traffic spikes.

So wait_timeout essentially frees connection handles when your application doesn't take care of them properly. There's no point that in a web application a connection should be idle for more than 10 seconds. And you don't want lots and lots of orphan connections to stand-by.

Secondly, the max_connections variable is important too because it's not enough to just crank it up to something like 5,000 – because while that means MySQL will allow that many connections, it will also allocate resources (RAM) to handle these potential connections – even if you never need them.

During the spikes, you should be able to connect to MySQL with your root account. That's a safety measure to be able to debug the server. My suggestion would be to also enable the slow-log temporarily.

In addition, during spikes check the processlist: mysqladmin -u root -pPASS PROCESSLIST. In case anything is cut off, connect with root (mysql -u root -pPASS) and issue SHOW FULL PROCESSLIST;.

From the process list, investigate queries which show up a couple times with EXPLAIN to get to the bottom of it. If they don't use an index, that's one of your problems right there.

Another option could be moving to something like Percona server. They have a lot of additions in there – the tip of the iceberg: xtradb (100% compatible to innodb) and a slow query log which gives you more granular output (milliseconds). It's free too, of course. A good read on all the things MySQL is their blog – the mysql performance blog.

LBNL – I'm just guessing, but it could be just lack of resources. c1.medium is a nice entry-level instance (t1.micro or m1.small serve no real purpose IM*H*O), but it could be not enough. It all depends on size of the database and actual traffic.

Feel free to comment and I can try to extend my answer.

Addition – I just read over the comments on the other answer.

You might want to get rid off the EBS-backed instance. I think they are a really bad idea. If you really need the persistence, you want to create a regular instance with ephemeral storage and then attach a couple (more than 1) EBS volume to it and RAID 10 across them for increase in IO/s.

Also, I have not mentioned that yet, but it also sounds like you lack monitoring on your server. Personally, we use Librato silverline, which provides us with near-realtime vitals on all of our instances. That should also help narrow down potential issues with the storage.

Answer 3

From just the symptoms mentioned, the following could be going on. I hope it helps.

It's possible that your PHP uses persistent database connections which perhaps don't get closed properly. Once a certain limit is reached, the database will not accept new connections anymore (from unix socket OR network).

In the php.ini there are settings related to database persistent connections, such as:

mysql.allow_persistent = Off

The fact that mysqld restarts don't work could be related to two things:

1. Restarting might not be the same as an explicit service mysqld stop followed by service mysqld start; also, you could check the logs while it restarts to see if it encounters anything unusual.

2. The restart sequence can be altered slightly to also involve your PHP setup, so you would stop apache first, then stop mysqld; afterwards, you start them in reverse order.