Error establishing a database connection - WordPress and mysql

Question

My WordPress website often has the error Error establishing a database connection .

Previously, I used sudo chmod 755 /var/lib/mysql and sudo /etc/init.d/mysql restart to fix it. However, this time, they could not work anymore; sudo /etc/init.d/mysql returned

stop: Unknown instance: 
start: Job failed to start

And mysqld --verbose returned

180120  3:24:48 [Warning] Using unique option prefix key_buffer instead of key_buffer_size is deprecated and will be removed in a future release. Please use the full name instead.
180120  3:24:48 [Warning] Can't create test file /var/lib/mysql/chengtie.lower-test
180120  3:24:48 [Note] mysqld (mysqld 5.5.58-0ubuntu0.14.04.1) starting as process 20724 ...
180120  3:24:48 [Warning] Can't create test file /var/lib/mysql/chengtie.lower-test
180120  3:24:48 [Warning] Can't create test file /var/lib/mysql/chengtie.lower-test
180120  3:24:48 [Warning] One can only use the --user switch if running as root

180120  3:24:48 [Warning] Using unique option prefix myisam-recover instead of myisam-recover-options is deprecated and will be removed in a future release. Please use the full name instead.
180120  3:24:48 [Note] Plugin 'FEDERATED' is disabled.
mysqld: Can't find file: './mysql/plugin.frm' (errno: 13)
180120  3:24:48 [ERROR] Can't open the mysql.plugin table. Please run mysql_upgrade to create it.
180120  3:24:48 InnoDB: The InnoDB memory heap is disabled
180120  3:24:48 InnoDB: Mutexes and rw_locks use GCC atomic builtins
180120  3:24:48 InnoDB: Compressed tables use zlib 1.2.8
180120  3:24:48 InnoDB: Using Linux native AIO
180120  3:24:48 InnoDB: Initializing buffer pool, size = 128.0M
InnoDB: mmap(137363456 bytes) failed; errno 12
180120  3:24:48 InnoDB: Completed initialization of buffer pool
180120  3:24:48 InnoDB: Fatal error: cannot allocate memory for the buffer pool
180120  3:24:48 [ERROR] Plugin 'InnoDB' init function returned error.
180120  3:24:48 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
180120  3:24:48 [ERROR] Unknown/unsupported storage engine: InnoDB
180120  3:24:48 [ERROR] Aborting

180120  3:24:48 [Note] mysqld: Shutdown complete

Does anyone know how to make my website back?

Answer 1

Your droplet does not have enough memory available to start MySQL. See the error InnoDB: mmap(137363456 bytes) failed; errno 12 InnoDB: mmap(137363456 bytes) failed; errno 12 - MySQL is failing when trying to allocate ~130MB of memory.
Run free -m to check how much memory is available to applications.

You can address this issue in two ways:

1. Increase the size of the droplet. This will cost you more money.
2. Add (more) swap space to your droplet. This will most probably impact performance but saves you money.