I have a mysql server used mostly as data warehouse. It loads about 4-5GB of data per day (actual database size, not the size of the source data).
Lately, I noticed something peculiar. After I stopped the loading process, and nothing else modifies the database, mysql keeps writing data at a rate of 5MB/s for over two hours.
This particular install uses 20GB of innodb_buffer_pool, so even if it was flushing all pages in memory, it would take about 70 minutes to do that.
Additional background information
This server is being mirrored, so it keeps the transaction log.
innodb_buffer_pool_pages_dirty keeps yoyo-ing between 60 and 10.
innodb_rows_(deleted|inserted|updated) is stable, so I'm pretty sure no modification is happening.
I have confirmed that it is indeed mysql doing the writting using
/proc/sys/vm/block_dump (see http://www.xaprb.com/blog/2009/08/23/how-to-find-per-process-io-statistics-on-linux/)
This last run, the server has only been up for two days, so it's highly unlikely that it has that many reads backlogged.
This is mysql 5.0 (yes it's old)