vscsiStats can be used to perform storage performance analyses. vscsiStats collects and reports counters on storage activity. Its data is collected at the virtual SCSI device level in the kernel. This means that results are reported per VMDK (or RDM) irrespective of the underlying storage protocol.
How to start it up
Open a putty session to your ESX server
Get help on vscsiStats: /usr/lib/vmware/bin/vscsiStats -h
Start collecting statistics (collection stops after 30 minutes): /usr/lib/vmware/bin/vscsiStats -s -w <worldgroupID>
To find the worldgroupID of the Virtual Machine you want to monitor: /usr/lib/vmware/bin/vscsiStats -l
Display counters: /usr/lib/vmware/bin/vscsiStats -p <histotype> -c
histotypes can be one of the following: all, ioLength, seekDistance, outstandingIOs, latency, interarrival
the option -c will use a comma as delimiter
See also http://communities.vmware.com/docs/DOC-10095