My question is: when is it appropriate to simply create one large array with fast read & write performance, such as RAID 10, instead of creating smaller arrays for specific applications?
It seems to me that if my overall I/O requirements are not terribly heavy, a single array with excellent read and write performance could provide better performance overall to all applications except at those (perhaps rare) times when applications with different access patterns peaked at the same time (copying large amounts of large files while a database is getting slammed).
If I dedicate a pair spindles to a particular task, such as transaction logs, and they're not even breaking a sweat with the workload...why not just put that workload onto a larger RAID 10? Those spindles would then be able to contribute to other workloads instead of sitting around scratching themselves 60% of the time.
PS, in my particular case the cost overhead of RAID 10 isn't a factor, because I'm looking at creating multiple RAID 1 arrays and one smallish RAID 5. Going RAID 10 for the amount of space I need would be comparable.