The first thing to note is that you get no error notifications when "service mysql start" fails -- just an indefinite hang, as this thread notes. If you become root and run /usr/sbin/mysqld, you will get an error like this:
100704 19:41:08 [Warning] The syntax '--log_slow_queries' is deprecated and will be removed in MySQL 7.0. Please use '--slow_query_log'/'--slow_query_log_file' instead.
100704 19:41:08 [Note] Plugin 'FEDERATED' is disabled.
/usr/sbin/mysqld: Can't create/write to file '/tmp/ib17ii5f' (Errcode: 13)
100704 19:41:09 InnoDB: Error: unable to create temporary file; errno: 13
100704 19:41:09 [ERROR] Plugin 'InnoDB' init function returned error.
100704 19:41:09 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
100704 19:41:09 [ERROR] Unknown/unsupported table type: innodb
100704 19:41:09 [ERROR] Aborting
100704 19:41:09 [Note] /usr/sbin/mysqld: Shutdown complete
errno 13 means "permission denied."
Because I was using an Amazon EBS root filesystem, I wanted to eliminate unnecessary EBS overhead, so I'd moved /tmp to /tmp-old, made the directory /mnt/tmp on the local storage, and symlinked /mnt/tmp to /tmp. I thought the perms were correct, and I was able to make files in /tmp as an unprivileged user. I even ran vipw to edit /etc/passwd to give the mysql user a shell (otherwise, you can't su to mysql) and noted that it was possible to write files to tmp. (I changed it back to /bin/false afterwards, of course.) Even when I told mysql to use the old directory ( /usr/sbin/mysqld --tmpdir=/tmp-old), I got the same error message.
I dug through the init files and noticed some references to app-armor. I checked the logs and found this:
Jul 4 19:41:09 domU-12-31-39-0E-C9-A1 kernel: [ 8095.267321] type=1503 audit(1278272469.041:18): operation="mknod" pid=5746 parent=4609 profile="/usr/sbin/mysqld" requested_mask="c::" denied_mask="c::" fsuid=106 ouid=106 name="/mnt/tmp/ib17ii5f"
The main reason for redirecting /tmp was an application of my own that produced thousands of cache files in /tmp. I changed the app to use /mnt/tmp instead. Now, mysql launched, but with a new error:
SSL error: Unable to get certificate from '/vol/etc/mysql/newcerts/server-cert.pem'
100704 19:52:14 [Warning] Failed to setup SSL
100704 19:52:14 [Warning] SSL error: Unable to get certificate
Unsurprisingly, in /var/log/messages, I found:
Jul 4 19:52:14 domU-12-31-39-0E-C9-A1 kernel: [ 8760.837526] type=1503 audit(1278273134.614:23): operation="open" pid=4609 parent=4602 profile="/usr/sbin/mysqld" requested_mask="r::" denied_mask="r::" fsuid=106 ouid=106 name="/vol/etc/mysql/newcerts/server-cert.pem"
The fix for this was pretty simple: I opened /etc/apparmor.d/usr.sbin.mysqld and below the line
I added the line