If you have a VPS, at some point you will have to make the upgrade from mysql 5.5 to 5.6. However the upgrade may fail if your VPS doesn't have more than 500MB of RAM or your all your free RAM resources are less than 400MB.
The reason is that mysql 5.6 needs to allocate 400MB additional RAM, because it has activated by default the mysql performance schema. This schema records statistics from the mysql database in runtime. Its main usage is to help the database administrator to check really low level measurements and identify performance issues like slow queries due to InnoDb locks, bottlenecks with Disk I/O etc.
To accomplish the upgrade you need to purchase additional memory for your VPS or to deactivate the performance schema. To deactivate it you will have to open the configuration file my.cnf. In any debian based distribution it usually is in:
After you open it with an editor like vim or nano you have to add in the basic settings under [mysqld] the following line:
performance_schema = off