Hyper-V harddisk consideration for exchange 2013 mailbox server role

Hi,

while i was going through videos on  virtualizing exchange server 2013 ,at some point it said  "With Mailbox role use SCSI pass-through disk or iSCSI" . What does it mean ?

We have virtualized our Exchange 2013 with 2 CAS  and Mailbox servers on Hyper-V.

We have created with default configurations like hard disk as IDE,will this affect the performance.

Thanks in advance.