zfs iscsi performance

Hi there! I've been playing for 4-6 weeks. The system is: ESXi 5.5 u2 + 4 NICS , round robin, FreeBSD 10.1 iscsi zfs I tried lot's of thing, read tons of http pages, however the sequential read speed is 50-60 MB\s from 6 HDD 7200rpm ( stripe of 3 mirrors ). zvol chunk = 4k NTFS allocation...