http://wiki.zimbra.com/wiki/Performance_Recommendations_for_Virtualizing_Zimbra_with_VMware_vSphere

iSCSI: https://marksallee.wordpress.com/2010/03/16/zimbra-mail-on-xenserver-virtual-machines-iscsi/

Optimization: http://wiki.zimbra.com/wiki/Performance_Tuning_Guidelines_for_Large_Deployments#JVM_Options

More Optimization: http://www.zoobey.com/index.php/2-rough/726-zimbra-optimization-part1

• NUMA - Size VM RAM no larger than RAM local to a single physical CPU
• Don't use memory ballooning on Zimbra VM
• 4GB RAM minimum - swaps a little
• 6GB RAM for a busier server or a VM

• Zimbra is not a CPU-bound workload
  • 2 vCPUs are generally recommended
  • Don't under-commit, over-commit or over-utilize vCPUs
  • Reduce vCPUs until peak usage points reach 60% utilization
• Periods of high, sustained CPU utilization can be caused by insufficient RAM or disk I/O wait
  • Try increasing the memory allocated to the VM

• In general, Zimbra is an I/O bound workload
• Optimize your storage in the normal ways
• iSCSI is recommended over NFS