Introduction

VERITAS Volume Manager (tm) 3.2 introduced the Device Discovery Layer (DDL) to allow easier support for third party arrays.  Volume Manager support for new disk arrays is provided in the form of a software package called Array Support Library (ASL).

The Device Discovery Layer (DDL) is a component of VERITAS Volume Manager that provides a device discovery service to the Volume Manager configuration daemon (vxconfigd). DDL is responsible for discovering the multipathing attributes of disks and of disk arrays that are connected to a host system. It also discovers any available enclosure information from the disk arrays.

DDL uses SCSI commands to discover disk array attributes. As the procedure for discovering such attributes differs between disk arrays, DDL is able to use a different dynamically loaded library for each specific type of disk array.  Such ASLs form the core modules for disk array support in Volume Manager.  After using ASLs to gather the attributes of all the disks connected to a host, DDL configures Volume Manager (specifically, the vxdmp driver) to enable multipathing of the disks.  

DDL allows ASLs to be added or removed from a running Volume Manager system. This means dynamic multipathing (DMP) support for a particular disk array type can be dynamically added to or removed from Volume Manager without stopping Volume Manager or rebooting the system. If the affected disks contain active volumes, they remain available during and after the addition or removal of support.

The administration utility vxddladm can be used to add or remove support for disk arrays, or to list supported disk arrays. It also supports dynamic reconfiguration of the set of array libraries that may participate in device discovery.  If the affected disk array contains active volumes, they remain available during and after the addition or removal of the ASL package.


Benefits of Using ASLs

Without ASLs,  Active/Passive arrays not already supported by Volume Manager will not be recognized.  If your array is currently not supported by Volume Manager,  it is recommended that you install the ASL package, if one is available.  To find out if your array is supported, use the vxddladm listsupport command.

In addition, some special functionality of an array may not be supported.  For example,  Hitachi arrays have a device called COMMAND DEVICE that is created for the host to communicate with the array.  Without the proper driver (ASL), Volume Manager does not filter the COMMAND DEVICE, so the device may be mistakenly used as a data disk. With the Hitachi ASL package installed, the Hitachi COMMAND DEVICE will be filtered and not used as a data disk. With the proper ASL package, the array will be identified as a Hitachi array and Volume Manager will utilize the full functionality of the Hitachi array.


Availability of ASLs

ASL packages may be provided by third party array vendors and may also be available on VERITAS Software CDs.

In addition to this, ASLs are available for download from the VERITAS Technical Support site,  http://support.veritas.com. Click on the Knowledge Base Search link, and select Volume Manager for UNIX from  e Product pull-down menu. Enter the search phrase "VERITAS Enabled Arrays" (quotes must   included in the search). Any newly issued ASLs can later be found at the same site.