Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

22 Understanding Veritas Volume Manager
How VxVM handles storage management
array, make up multiple hardware paths to access the disk devices. Such disk arrays are
called multipathed disk arrays. This type of disk array can be connected to host systems in
many different configurations, (such as multiple ports connected to different controllers
on a single host, chaining of the ports through a single controller on a host, or ports
connected to different hosts simultaneously).For more detailed information, see
“Administering dynamic multipathing (DMP)” on page 121.
Device discovery
Device discovery is the term used to describe the process of discovering the disks that are
attached to a host. This feature is important for DMP because it needs to support a
growing number of disk arrays from a number of vendors. In conjunction with the ability
to discover the devices attached to a host, the Device Discovery service enables you to add
support dynamically for new disk arrays. This operation, which uses a facility called the
Device Discovery Layer (DDL), is achieved without the need for a reboot.
This means that you can dynamically add a new disk array to a host, and run a command
which scans the operating system’s device tree for all the attached disk devices, and
reconfigures DMP with the new device database. For more information, see
“Administering the Device Discovery Layer” on page 85.
Enclosure-based naming
Enclosure-based naming provides an alternative to the disk device naming described in
“Physical objects—physical disks” on page 20. 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 hubs or fabric 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.
In a typical SAN environment, host controllers are connected to multiple enclosures in a
daisy chain or through a Fibre Channel hub or fabric switch as illustrated in Figure 1-3.