One of the most interesting new vSphere storage features in my opinion is the new virtual
disk paravirtualized SCSI (PVSCSI) controller. It has been reported *that improved I/O with as much as 18%
reduction in ESX 4 host CPU usage can be achieved by switching to
PVSCSI*. The benefits of PVSCSI performance are twofold:
- Reduced data center power and cooling costs to when you consider the impact
of tens of hosts not having to work as hard - A potential higher VM to host consolidation ratio when more CPU cycles are
available
For reference, EMC virtualization guru Chad Sakac provided a post that
explains the PVSCSI performance benefits:
http://virtualgeek.typepad.com/virtual_geek/2009/05/update-on-the-io-vsphere-performance-test.html
However, *to take advantage of PVSCSI a VM virtual disk configuration
might need to change. Because VMware does not support PVSCSI on the operating
system boot partition, VMs will need to be configured with separate virtual
disks(.vmdk) for the boot drive and the data drive(s)*. Note that all
the posts and articles referenced mention that PVSCSI works on a .vmdk
containing the boot partition. It’s just that VMware officially does not support
it.
So, *the challenge for using PVSCSI then is to migrate services and
applications that exist on VMs that contain both the boot partition and the data
on a single .vmdk*. Although separate boot and data partitions are the
defacto standard for physical servers, the convenience of VMs has lead to a single .vmdk configuration in a lot of IT shops.
Incentive to use PVSCSI therefore actually overlaps with a shift in VM
deployment strategy and ultimately supports and provides performance reasons to
adopt smaller, dedicated .vmdks for boot partitions. This multi .vmdk design
change also has other benefits including optimization of deduplication and DR
site replication technologies.
Here are some quick thoughts on deploying and migrating VMs to a multiple
.vmdk configuration.
- Build a golden image VM template with multiple .vmdks, or change future VM
deployment policy to include adding new .vmdks for installing applications and
storing data. - For VMs that already have separate partitions on a single .vmdk use VMware
Converter or another tool to V2V to a new VM with separate .vmdks for each
partition. - When possible, make sure P2V migrations of physical servers result in a
separate .vmdk for each partition - Unfortunately, building new VMs and reinstalling the applications may be the
only choice for existing implementations combined on a single partition .vmdk
Once the .vmdk configuration is ready, *PVSCSI can be enabled
following the processes explained in these posts*:
- Adding the PVSCSI adapter
in vSphere – - For Windows
VMs – - For Linux
VMs –
The final question may be “is it worth the effort to migrate to a PVSCSI supporting
configuration for all VMs”? The performance, consolidation, and cost savings
factors would lead most virtual administrators to answer “yes”, but ultimately
the decision will most likely be made on a VM by VM basis. There are some other
factors to consider as well. For example, vSphere Fault Tolerance cannot be
enabled on a VM using PVSCSI.
VMware’s PDF on the new vSphere storage features can be found at http://www.vmware.com/files/pdf/VMW_09Q1_WP_vSphereStorage_P10_R1.pdf.
