vxdmp - Veritas Volume Manager multipathing disk devices

Multipathed disk devices in Veritas Volume Manager (VxVM) represent virtual devices with one or more physical access paths to a physical disk. The Multipathing feature provides disk access reliability by dynamically switching to another physical path if there is a path failure.

Every disk that is visible to VxVM is represented by a multipath device node in the /dev/vx/dmp and /dev/vx/rdmp directories for block type and character type interfaces. A simple disk is represented by a multipath device having one physical path, whereas a disk that is part of a multiported disk array is represented by a multipath device with many device access paths depending upon the type of the disk array.

VxVM uses standard disk device naming conventions to name multipath devices. For example, if there is a disk device with two physical access paths such as sdf and sdn, the multipathing device node for this particular disk is created under /dev/vx/dmp and /dev/vx/rdmp as sdf or sdn.

To provide multipathing, VxVM maintains an internal database of all the multipathed devices detected by the system and their respective paths. The database is created at system boot time. When disks are added or removed from the system, VxVM provides support to reconfigure the multipath device database to reflect new states of the hardware, while the system is online.

The various access paths to a multipathed device can be seen by using a vxdisk command (see vxdisk(1M)). These commands open the multipathing configuration device /dev/vx/dmpconfig to obtain the required information about the various paths to a multipathed device and their states.

VxVM also provides the facility to enable or disable a disk controller. This feature can be used to stop all I/Os through a particular controller to perform maintenance operations on it. Use the vxdmpadm command to do this and also perform other operations on the multipath device database.

VxVM can provide notification about events that occur in the DMP database due to a change in hardware. See the vxnotify(1M) man page for more information.

VxVM is capable of multipathing both Active/Active and Active/Passive disk arrays.

The ioctl commands supported by the multipathing device interface are:

DMP_GET_SUBPATHS
	Find the various subpaths of a multipathing device.
DMP_GET_MULTIPATH
	Find the multipathing device for a given subpath.
DMP_CHANGE_CTLR_STATE
	Change the state of a controller.
DMP_GET_CTLRS
	List all disk controllers on the system.
DMP_GET_PATHS_THRU_CTLR
	Find all paths going through a controller.

/dev/vx/dmp	Multipathing block devices.
/dev/vx/rdmp	Multipathing character devices.

Do not invoke the ioctl commands directly. An API for all the functionality is available in the VxVM developer’s package.

ioctl(2), vxdctl(1M), vxdisk(1M), vxdiskconfig(1M), vxdmpadm(1M), vxnotify(1M)