DDL may not be able to discover some devices that are controlled by third-party drivers, such as those that provide multi-pathing or RAM disk capabilities. For these devices it may be preferable to use the multi-pathing capability that is provided by the third-party drivers for some arrays rather than using Dynamic Multi-Pathing (DMP). Such foreign devices can be made available as simple disks to VxVM by using the vxddladm addforeign command. This also has the effect of bypassing DMP for handling I/O. The following example shows how to add entries for block and character devices in the specified directories:
# vxddladm addforeign blockdir=/dev/foo/dsk \ chardir=/dev/foo/rdsk
By default, this command suppresses any entries for matching devices in the OS-maintained device tree that are found by the autodiscovery mechanism. You can override this behavior by using the -f and -n options as described on the vxddladm(1M) manual page.
After adding entries for the foreign devices, use either the vxdisk scandisks or the vxdctl enable command to discover the devices as simple disks. These disks then behave in the same way as autoconfigured disks.
The foreign device feature was introduced in VxVM 4.0 to support non-standard devices such as RAM disks, some solid state disks, and pseudo-devices such as EMC PowerPath.
Foreign device support has the following limitations:
A foreign device is always considered as a disk with a single path. Unlike an autodiscovered disk, it does not have a DMP node.
It is not supported for shared disk groups in a clustered environment. Only standalone host systems are supported.
It is not supported for Persistent Group Reservation (PGR) operations.
It is not under the control of DMP, so enabling of a failed disk cannot be automatic, and DMP administrative commands are not applicable.
Enclosure information is not available to VxVM. This can reduce the availability of any disk groups that are created using such devices.
The I/O Fencing and Cluster File System features are not supported for foreign devices.
If a suitable ASL is available and installed for an array, these limitations are removed.