Partition alignment and NTFS Allocation unit size

Below is a table of recommendations for use with Exchange.

Best practice: 64 KB for both .edb and log file volumes.

Description	Recommended Value
Storage Track Boundary	64K or greater. (1MB recommended)
NTFS allocation unit/cluster size	64KB (DB and Log Drives)
RAID Stripe size	256KB or greater. Check with your storage vendor for best practices

Source : http://blogs.technet.com/b/mikelag/archive/2011/02/09/how-fragmentation-on-incorrectly-formatted-ntfs-volumes-affects-exchange.aspx

Plus : http://technet.microsoft.com/fr-fr/library/ee832792.aspx