More key buffer used than sum of all .MYI files

[Daniel Nichter]

My understanding of the key buffer is that it is only used for indexes. As such, it could possibly be as large as all .MYI files combined (but is usually lower due to on-demand caching). If that is accurate, I've run into a strange situation: A server with 65M total .MYI files, but 256M key buffer used. It starts off 3M key buffer used and creeps up slowly to 256M every time (takes less than a day to reach this). What could be causing this? Or is the key buffer used for other things? Thank you.