index IO patterns

Hey all, we're in the process of checking out alternatives to our index storage.  We're currently storing
indexes on a NetApp Metrocluster which works fine, but is very expensive. We're planning a few different
setups and doing some actual performance tests on them.  

Does anyone know some of the IO patterns of the indexes? For instance:

- mostly random reads or linear reads/writes? 
- average size of reads and writes?
- how many read/writes on average for a specific mailbox size?

Anyone do any measurements of this kind?

Alternatively, does anyone have any experience with other redundant storage options? Im thinking things
like MooseFS, DRBD, etc? 

regards,

Cor