How to monitor InnoDB Disk I/O Required for MySQL?
Innodb_buffer_pool_read_requests and Innodb_buffer_pool_reads are the values you need to monitor for buffer reads and disk read for the Buffer Pool.
What you are looking for is a cache miss rate of 1% or less based on this
Innodb_buffer_pool_reads X 100 / Innodb_buffer_pool_read_requests

I have written posts like this before

Feb 04, 2014 : Tuning key_reads in MySQL
Jun 08, 2012 : How to improve MySQL Server Performance..??

You should also be tracking how much data is being poured into the InnoDB transaction logs

Innodb_os_log_written : The number of bytes written to the InnoDB redo log files.
Innodb_log_write_requests : The number of write requests for the InnoDB redo log.
Innodb_log_writes : The number of physical writes to the InnoDB redo log file.

EXAMPLE
you could take the delta of Innodb_os_log_written (bytes written in a one second window) and divide it by the delta of Innodb_log_writes and track how many bytes are being flushed to the logs per second. If the result seems too high for you, you could tweek innodb_flush_method, innodb_flush_log_at_trx_commit, innodb_log_file_size and things of this nature.