In order to provide a reliable connection between VMware ESX hosts and their disk storage, multiple paths through the SAN are required. This is accomplished by using multiple host bus adapters (HBAs) on the host, multiple ports on the disk subsystem, and redundant routing through the switched SAN fabric. ESX provides support to understand and exploit the resulting multiple paths between host and storage.
To maintain integrity during updates by way of multiple paths, locking mechanisms are required to control access. When performing updates (such as creating a new virtual machine), VMware ESX will utilize SCSI-2 reserve/release commands for LUN-level locking. This has the potential to reduce I/O performance if reservation conflicts occur.
This IBM® Redpapers™ publication will explain the variations in multipathing support for different types of storage controller, and explain where reservation based locking is used.
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