Depending on the SAN it may make sense for e.g. an SQL server to put the DB's and the transaction logs on different LUNs. However IMO there's no benefit in separating the OS and the data disks, because the OS usually does not have a high IO (reads/writes).
If the LUNs are both using the same path to the array then the only win is that you have more IO Queues available within the stack. This is why Queudepth is a pretty major factor with FC HBAs. Other than there is no major win if the LUNs are using the same wire. However, if they are load balanced across multiple wires, then yes there is a chance for improvement.
It really depends on your networking setup for iSCSI. Aggregate means many things, and since ESX by default does not 'aggregate' traffic but load balances, you need to know on which wire the LUN is writing/reading......
Edward L. Haletky VMware Communities User Moderator, VMware vExpert 2009, 2010
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