VDisk-to-host mapping

Virtual disk (VDisk)-to-host mapping is the process of controlling which hosts have access to specific VDisks within the SAN Volume Controller cluster.

VDisk-to-host mapping is similar in concept to logical unit number (LUN) mapping or masking. LUN mapping is the process of controlling which hosts have access to specific logical units (LUs) within the disk controllers. LUN mapping is typically done at the disk controller level. VDisk-to-host mapping is done at the SAN Volume Controller level.

The act of mapping a VDisk to a host makes the VDisk accessible to the WWPNs or iSCSI names such as iSCSI qualified names (IQNs) or extended-unique identifiers (EUIs) that are configured in the host object.

VDisks and host mappings

Each host mapping associates a VDisk with a host object and allows all WWPNs and iSCSI names in the host object to access the VDisk. You can map a VDisk to multiple host objects. When a mapping is created, multiple paths might exist across the SAN fabric or Ethernet network from the hosts to the SAN Volume Controller nodes that are presenting the VDisk. Without a multipathing device driver, most operating systems present each path to a VDisk as a separate storage device. The multipathing software manages the many paths that are available to the VDisk and presents a single storage device to the operating system. If there are multiple paths, the SAN Volume Controller requires that the multipathing software run on the host.

Note: The iSCSI names and associated IP addresses for the SAN Volume Controller nodes can fail over between nodes in the I/O group, which negates the need for multipathing drivers in some configurations. Multipathing drivers are still recommended, however, to provide the highest availability.

When you map a VDisk to a host, you can optionally specify a SCSI ID for the VDisk. This ID controls the sequence in which the VDisks are presented to the host. Check the host software requirements for SCSI IDs because some require a contiguous set. For example, if you present three VDisks to the host, and those VDisks have SCSI IDs of 0, 1, and 3, the VDisk that has an ID of 3 might not be found because no disk is mapped with an ID of 2. The cluster automatically assigns the lowest available SCSI ID if none is specified.

Figure 1 and Figure 2 show two VDisks, and the mappings that exist between the host objects and these VDisks.

Figure 1. Hosts, WWPNs, IQNs or EUIs, and VDisks

This figure is described in the surrounding text.

Figure 2. Hosts, WWPNs, IQNs or EUIs, VDisks, and SCSI mappings

LUN masking is usually implemented in the device driver software on each host. The host has visibility of more LUNs than it is intended to use, and device driver software masks the LUNs that are not to be used by this host. After the masking is complete, only some disks are visible to the operating system. The SAN Volume Controller can support this type of configuration by mapping all VDisks to every host object and by using operating system-specific LUN masking technology. The default, and recommended, SAN Volume Controller behavior, however, is to map to the host only those VDisks that the host requires access to.