MySQL server hangs without reason

You probably shouldn't consider MyISAM, INNODB will work for you. MyISAM is maybe faster when it comes to SELECT but (for example) it locks your full table on updates.

As for INNODB:

• generally, always consider more RAM before you go into sharding (size of the DB =~ RAM)
• take a look at the following variables:
  • innodb_buffer_pool_size (we use roughly 60-70% of our memory)
  • innodb_log_file_size
  • innodb_log_buffer_size
  • innodb_flush_log_at_trx_commit
  • innodb_thread_concurrency
  • innodb_flush_method=O_DIRECT
  • innodb_file_per_table
• switch from innodb to xtradb (same API)
• use the percona builds (they contain performance patches from Google, etc.)

Great reads:

• innodb performance optimization basics
• choosing innodb buffer pool size
• innodb tuning (mysql man)

On a side note:

• a 2 GB slice is just not enough to run this
• further more I found the storage on slicehost to be rather slow (io is a factor)
• in the cloud it may make sense to shard earlier (cause of the RAM limit)
• I'd run all queries through EXPLAIN to make sure the index is really being used

Check the values of your mysql server. Are you using default users in mysql? I'd also take a look at two things:

1. Be sure that only specific servers can connect to the mysql database. If you have a user that is connecting from % be sure that you have a good password (and good reason).

2. Be sure that someone isn't sending queries to your application outside of the code requirements that you have in place to run the application. Someone may be injecting data into your database without even visiting your site.

Keep track of you active connections for a while by paying close attention to netstat. The wait_timeout is an indication that someone/something has queried your server. Keeping close track will help you discover who they are. Once you do that if you need to quickly block them without making drastic changes, use iptables.

One last thing... enable mysql logging briefly. Your server will take a hit but you'll see exactly what's going on with mysql.

edit the my.cnf file and add (or update) this:

log             = /var/log/mysql/mysql.log

EDIT: I made the preceding suggestions because the NULL value in the info field indicates that no statements are being executed.