Based on MySQL 5.5.21, including all the bug fixes in it, Percona Server 5.5.21-25.0 is now the current stable release in the 5.5 series. All of Percona‘s software is open-source and free, all the details of the release can be found in the 5.5.21-25.0 milestone at Launchpad.
Improvements to the XtraDB’s sync flush algorithm made in Percona Server 5.5.19-24.0 have been reverted because of the performance issues on SSDs (Laurynas Biveinis).
Slow query logging features have been expanded by adding new variable log_slow_rate_type. It now provides option to specify the query sampling being taken. If the variable is set up to “query”, sampling is done on per query basics instead of session, which is the default (Oleg Tsarev).