[ERROR] Plugin ‘InnoDB’ init function returned error.

I was getting below mysql error log:-

[Note] Plugin 'FEDERATED' is disabled.
InnoDB: The InnoDB memory heap is disabled
InnoDB: Mutexes and rw_locks use GCC atomic builtins
InnoDB: Compressed tables use zlib 1.2.3
InnoDB: Using Linux native AIO
InnoDB: Initializing buffer pool, size = 128.0M
InnoDB: mmap(137363456 bytes) failed; errno 12
InnoDB: Completed initialization of buffer pool
InnoDB: Fatal error: cannot allocate memory for the buffer pool
[ERROR] Plugin 'InnoDB' init function returned error.
[ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
[ERROR] Unknown/unsupported storage engine: InnoDB
[ERROR] Aborting
[Note] /usr/libexec/mysqld: Shutdown complete

I found out there are two solutions which are:-

1)Set innodb_log_file_size equal to the actual size of the existing InnoDB log files. To see what size of innoDB log allocated, login mysql and enter following cmd:-

SHOW GLOBAL VARIABLES LIKE 'innodb_log_file_size';

Expected result example:- 5242880

After that, insert that value in my.cnf:- vi /etc/my.cnf

 innodb_log_file_size =5242880

2)Rename or move both the ./ib_logfile0 and ./ib_logfile1 files, and then start the MySQL server.This normally will be located at /var/lib/mysql. After start mysql, it create new innoDB log file and restore possible half-written data from the file of .ibd.

The expexted mysql log example:-

InnoDB: Database physically writes the file full: wait...
161216  9:58:54  InnoDB: Log file ./ib_logfile1 did not exist: new to be created
InnoDB: Setting log file ./ib_logfile1 size to 5 MB
InnoDB: Database physically writes the file full: wait...
161216  9:58:54 InnoDB: highest supported file format is Barracuda.
InnoDB: The log sequence number in ibdata files does not match
InnoDB: the log sequence number in the ib_logfiles!
161216  9:58:54  InnoDB: Database was not shut down normally!
InnoDB: Starting crash recovery.
InnoDB: Reading tablespace information from the .ibd files...
InnoDB: Restoring possible half-written data pages from the doublewrite
InnoDB: buffer...
161216  9:58:54  InnoDB: Waiting for the background threads to start
161216  9:58:55 InnoDB: 5.5.50 started; log sequence number 1589772
161216  9:58:55 [Note] Server hostname (bind-address): ''; port: 3306
161216  9:58:55 [Note]   - '' resolves to '';
161216  9:58:55 [Note] Server socket created on IP: ''.
161216  9:58:55 [Note] Event Scheduler: Loaded 0 events
161216  9:58:55 [Note] /usr/libexec/mysqld: ready for connections.
Version: '5.5.50'  socket: '/var/lib/mysql/mysql.sock'  port: 3306  MySQL Community Server (GPL) by Remi

References:- JUSTIN KULESZA (2011). MySQL: Failed Registration of InnoDB as a Storage Engine. Available at: https://spin.atomicobject.com/2011/05/09/mysql-failed-registration-of-innodb-as-a-storage-engine/.

RolandoMySQLDBA (2014). MySQL my.cnf: innodb_log_file_size is missing. Available at: http://dba.stackexchange.com/questions/75688/mysql-my-cnf-innodb-log-file-size-is-missing/158325#158325

Changing the Number or Size of InnoDB Redo Log Files. Available at: http://dev.mysql.com/doc/refman/5.7/en/innodb-data-log-reconfiguration.html