Chapter 9 Managing Storage

What to do next

After resignaturing, you might have to do the following:

nIf the resignatured datastore contains virtual machines, update references to the original VMFS datastore in the virtual machine files, including .vmx, .vmdk, .vmsd, and .vmsn.

nTo power on virtual machines, register them with vCenter Server.

Using Multipathing with ESX

To maintain a constant connection between an ESX host and its storage, ESX supports multipathing. Multipathing is a technique that lets you use more than one physical path for transferring data between the ESX host and the external storage device.

In case of a failure of an element in the SAN network, such as an HBA, switch, or cable, ESX can use alternate physical paths to access a storage device. This process is known as path failover. In addition to path failover, multipathing offers load balancing, which redistributes I/O loads between multiple paths, thus reducing or removing potential bottlenecks.

Managing Multiple Paths

To manage storage multipathing, ESX uses a special VMkernel layer, the Pluggable Storage Architecture (PSA). The PSA is an open, modular framework that coordinates the simultaneous operation of multiple multipathing plug-ins (MPPs).

The VMkernel multipathing plug-in that ESX provides by default is the VMware Native Multipathing PlugIn (NMP). The NMP is an extensible module that manages sub plug-ins. There are two types of NMP sub plugins, Storage Array Type Plug-Ins (SATPs), and Path Selection Plug-Ins (PSPs). SATPs and PSPs can be built-in and provided by VMware, or can be provided by a third party.

If more multipathing functionality is required, a third party can also provide an MPP to run in addition to, or as a replacement for, the default NMP.

When coordinating the VMware NMP and any installed third-party MPPs, the PSA performs the following tasks:

nLoads and unloads multipathing plug-ins.

nHides virtual machine specifics from a particular plug-in.

nRoutes I/O requests for a specific logical device to the MPP managing that device.

nHandles I/O queuing to the logical devices.

nImplements logical device bandwidth sharing between virtual machines.

nHandles I/O queueing to the physical storage HBAs.

nHandles physical path discovery and removal.

nProvides logical device and physical path I/O statistics.

As Figure 9-1 illustrates, multiple third-party MPPs can run in parallel with the VMware NMP. When installed, the third-party MPPs replace the behavior of the NMP and take complete control of the path failover and the load-balancing operations for specified storage devices.