When downgrading to MySQL 5.1 from MySQL 5.5, you should keep in mind the following issues relating to features found in MySQL 5.5, but not in MySQL 5.1:
MySQL 5.5 uses
InnoDB Plugin as the
built-in version of
Plugin as of 5.1.38, but as an option that must
be enabled explicitly. See the
Notes for MySQL 5.1.38.
Tables partitioned by
MySQL 5.5 implements new functions used to calculate row
placement for tables partitioned by
LINEAR KEY. Tables that were
KEY partitioning in MySQL 5.5 cannot be used by
a MySQL 5.1 server. In MySQL 5.5.31 and later, you can
downgrade such tables with
TABLE ... PARTITION BY ALGORITHM=1 [LINEAR] KEY
(...) to make them compatible with MySQL 5.1.