Q: What's VMware’s PVSCSI driver?

A: PVSCSI is the VMware Paravirtual SCSI driver. This driver is designed for use in virtual machines (VMs) with very high storage performance requirements. According to VMware knowledgebase article 1010398 (found at http://kb.vmware.com/kb/1010398), the PVSCSI driver is "best suited for environments, especially SAN environments, where hardware or applications drive a very high amount of I/O throughput." VMware suggests this adapter isn't intended for DAS storage.

The PVSCSI driver might increase storage performance for VMs with heavy storage utilization across SAN connections, but it does so with a set of limitations. Found in the same knowledgebase article, those limitations are:

  • Hot add or hot remove requires a bus rescan from within the guest.
  • Disks with snapshots might not experience performance gains when used on Paravirtual SCSI adapters or if memory on the ESX host is overcommitted.
  • If you upgrade from RHEL 5 to an unsupported kernel, you might not be able to access data on the virtual machine's PVSCSI disks. You can run vmware-config-tools.pl with the kernel-version parameter to regain access.

Be cautious and test thoroughly prior to implementing VMware’s PVSCSI driver. Some OSs aren't supported, the limitations above may have an impact on VM operations, and some workloads won't see a measurable performance benefit.

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