Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

How DMP Works
106 VERITAS Volume Manager Administrator’s Guide
Coexistence of DMP with Native Multipathing in HP-UX 11i v3
HP-UX 11i v3 provides native multipathing as part of its mass storage stack. Two kinds of
device special file are supported:
◆ Legacy device special files in the /dev/dsk and /dev/rdsk directories. These
device files are only created for the first 32,768 paths on a system. Such device files can
be discovered by DMP, and coexistence is supported. DMP works as a pass-through
driver, and directs I/O and ioctl requests on one path without any load balancing.
Native multipathing in HP-UX 11i v3 performs all multipathing and load balancing.
◆ Device special files in the /dev/disk and /dev/rdisk directories. One device file is
created to represent each native multipathing metanode. These device files are created
for all paths in a system, and more than 32,768 paths are supported. Such device files
are not discovered by DMP, and coexistence is not supported.
During device discovery, VxVM creates entries in the DMP database for the relationships
between DMP nodes and the legacy device paths. Each DMP node represents a set of
legacy paths. The legacy paths map internally to a native multipathing metanode that
controls a set of physical paths.
Native multipathing in HP-UX 11i v3 takes care of load balancing, failover and failback
for the paths that it controls. The default I/O policy for DMP is set to singleactive for
all types of enclosure. When DMP receives or sends I/O on a legacy path, native
multipathing chooses the appropriate physical path. Unless all the underlying physical
paths of the native multipathing metanode fail, any I/O that DMP performs on a legacy
path succeeds.
See “Specifying the I/O Policy” on page 121.
See “Displaying All Paths Controlled by a DMP Node” on page 114.
Note the following limitations of DMP coexistence with HP-UX native multipathing:
◆ DMP supports the A/P arrays that are supported by HP-UX native multipathing. No
additional A/P arrays are supported. If an A/P array is used that is not supported by
HP-UX native multipathing, any I/O or ioctl requests from DMP cause trespass.
◆ A path-specific attribute that is shown by the DMP vxdmpadm getsubpaths
command may not match the actual array attribute. This is because DMP cannot
perform path-specific I/O or ioctl requests unless legacy multipathing is disabled.
Instead DMP shows the path attributes that are returned by HP-UX native
multipathing. For example, on an A/P array, all paths may be shown as PRIMARY or
SECONDARY.
◆ For A/A arrays, the vxdmpadm getsubpaths command shows all paths in the
ENABLED (A) state until the last path is disabled. For arrays of type A/P, only one
path is shown in the ENABLED (A) state. All path types are shown as PRIMARY until