Veritas™ Volume Manager Administrator's Guide

About enclosure-based naming

Enclosure-based naming provides an alternative to operating system-based device naming. This allows disk devices to be named for enclosures rather than for the controllers through which they are accessed. In a Storage Area Network (SAN) that uses Fibre Channel switches, information about disk location provided by the operating system may not correctly indicate the physical location of the disks. For example, c#t#d# naming assigns controller-based device names to disks in separate enclosures that are connected to the same host controller. Enclosure-based naming allows VxVM to access enclosures as separate physical entities. By configuring redundant copies of your data on separate enclosures, you can safeguard against failure of one or more enclosures.

Figure: Example configuration for disk enclosures connected via a fibre channel switch shows a typical SAN environment where host controllers are connected to multiple enclosures through a Fibre Channel switch.

Figure: Example configuration for disk enclosures connected via a fibre channel switch

In such a configuration, enclosure-based naming can be used to refer to each disk within an enclosure. For example, the device names for the disks in enclosure enc0 are named enc0_0, enc0_1, and so on. The main benefit of this scheme is that it allows you to quickly determine where a disk is physically located in a large SAN configuration.

In most disk arrays, you can use hardware-based storage management to represent several physical disks as one LUN to the operating system. In such cases, VxVM also sees a single logical disk device rather than its component disks. For this reason, when reference is made to a disk within an enclosure, this disk may be either a physical disk or a LUN.

Another important benefit of enclosure-based naming is that it enables VxVM to avoid placing redundant copies of data in the same enclosure. This is a good thing to avoid as each enclosure can be considered to be a separate fault domain. For example, if a mirrored volume were configured only on the disks in enclosure enc1, the failure of the cable between the switch and the enclosure would make the entire volume unavailable.

If required, you can replace the default name that VxVM assigns to an enclosure with one that is more meaningful to your configuration.

Figure: Example HA configuration using multiple switches to provide redundant loop access shows a High Availability (HA) configuration where redundant-loop access to storage is implemented by connecting independent controllers on the host to separate switches with independent paths to the enclosures.

Figure: Example HA configuration using multiple switches to provide redundant loop access

Such a configuration protects against the failure of one of the host controllers (c1 and c2), or of the cable between the host and one of the switches. In this example, each disk is known by the same name to VxVM for all of the paths over which it can be accessed. For example, the disk device enc0_0 represents a single disk for which two different paths are known to the operating system, such as c1t99d0 and c2t99d0.

To take account of fault domains when configuring data redundancy, you can control how mirrored volumes are laid out across enclosures.