Before (but not necessarily at the same time as) a compressed page is written to a database file, InnoDB writes a copy of the page to the redo log (if it has been recompressed since the last time it was written to the database). This is done to ensure that redo logs will always be usable, even if a future version of InnoDB uses a slightly different compression algorithm. Therefore, some increase in the size of log files, or a need for more frequent checkpoints, can be expected when using compression. The amount of increase in the log file size or checkpoint frequency depends on the number of times compressed pages are modified in a way that requires reorganization and recompression.
Note that the redo log file format (and the database file format) are different from previous releases when using compression. The current release of InnoDB Hot Backup (version 3) therefore does not support databases that use compression. Only databases using the file format “Antelope” can be backed up online by InnoDB Hot Backup.
This is the User’s Guide for InnoDB storage engine 1.1 for MySQL 5.5, generated on 2010-04-13 (revision: 19994) .