When installing ESXi and enable iSCSI datastores with multipath round robin the performance could be poor. VMware will use the other path if it reaches 1000 IOPS. This is the default setting.

So I have done some measurements.

iSCSI FreeNAS 9.3 zpool status:

  pool: sata-disk
 state: ONLINE
  scan: scrub repaired 0 in 2h59m with 0 errors on Sun May  3 09:59:59 2015
config:

	NAME                                            STATE     READ WRITE CKSUM
	sata-disk                                       ONLINE       0     0     0
	  mirror-0                                      ONLINE       0     0     0
	    gptid/f8c7b55e-4ef0-11e4-98c0-0cc47a0917b6  ONLINE       0     0     0
	    gptid/f92b9f6b-4ef0-11e4-98c0-0cc47a0917b6  ONLINE       0     0     0
	  mirror-2                                      ONLINE       0     0     0
	    gptid/1558a0b6-4ef2-11e4-98c0-0cc47a0917b6  ONLINE       0     0     0
	    gptid/15b29c40-4ef2-11e4-98c0-0cc47a0917b6  ONLINE       0     0     0
	logs
	  mirror-1                                      ONLINE       0     0     0
	    da0p1                                       ONLINE       0     0     0
	    da4p1                                       ONLINE       0     0     0
	cache
	  da4p2                                         ONLINE       0     0     0
	  da0p2                                         ONLINE       0     0     0
	spares
	  gptid/fac4aa65-4ef0-11e4-98c0-0cc47a0917b6    AVAIL   

errors: No known data errors