I am thinking about converting some tables from MyISAM to InnoDB in my mysql server. The tables will certainly benefit from the change because a lot of write requests come to these tables, while there are also quite a lot of read request at the same time.

However, they are often joined together with some tables that almost don't get any writes. Is there a performance penalty when joining together MyISAM and InnoDB tables or should everything work fine?

Second question: During backups at night, I am copying data from the InnoDB tables to MyISAM tables for archiving purposes. In these backups, a lot of write-requests happen, however there is almost no read from these archive tables. Would these tables also benefit from using InnoDB or is this just a waste of space and RAM?