What are some good methods of structuring a MySQL audit trail to be both immutable, and perform well for usage?

In one application, we have an audit table that uses the ArchiveEngine in conjunction with a few triggers to capture 100% of the changes that occur. The upside to this is that 20+ million rows occupies 145MB, not to mention that ArchiveEngine only supports INSERT and SELECT, rendering the trail immutable. The downside comes when you want to access that data in order to give the user audit information. Since the data is stored in compressed form and there are no indices, selection time can be ridiculous: mysql> select * from AUDIT_LOG WHERE Table_Name = 'myTable' AND Row_ID = 9024; +------------+--------+-------------+---------------------+---------------------+----------------+-------------+---------------------+ | Table_Name | Row_ID | Field_Name | Old_Value | New_Value | DB_User | modified_by | date_modified | +------------+--------+-------------+---------------------+---------------------+----------------+-------------+---------------------+ | myTable | 9024 | Flags | 0 | 1 | user@localhost | 826 | 2011-06-10 22:05:11 | | myTable | 9024 | Status | Unavailable | Pending | user@localhost | 826 | 2011-07-08 22:41:45 | | myTable | 9024 | Status | Pending | Processing | user@localhost | 826 | 2011-07-08 22:41:49 | | myTable | 9024 | Status | Processing | Calculated | user@localhost | 826 | 2011-07-08 22:41:54 | | myTable | 9024 | Fee_Paid | NULL | 185.00 | user@localhost | 826 | 2011-07-08 22:41:54 | | myTable | 9024 | Status | Calculated | Approved | user@localhost | 826 | 2011-07-08 22:47:04 | | myTable | 9024 | Approved | NULL | 2011-07-08 22:47:06 | user@localhost | 826 | 2011-07-08 22:47:04 | | myTable | 9024 | Approved_TZ | America/Yellowknife | America/Vancouver | user@localhost | 826 | 2011-07-08 22:47:04 | | myTable | 9024 | Approved_By | NULL | 826 | user@localhost | 826 | 2011-07-08 22:47:04 | | myTable | 9024 | Invoice_ID | NULL | 2372 | user@localhost | 825 | 2011-10-17 19:32:59 | +------------+--------+-------------+---------------------+---------------------+----------------+-------------+---------------------+ **10 rows in set (13.54 sec)** After conversion to InnoDB and adding a PK and indexing Row_ID and Table_Name, this same operation takes roughly 1/10th of a second. What I'm considering is a leaving the triggers + Archive, and then using a cron job to mirror the Archive in InnoDB. - Are there other ways this has been dealt with? - Any drawbacks or trade-offs that are apparent?