HP StorageWorks SAN Virtualization Services Platform Administrator Guide (5697-0934, May 2011)

The DPM has a back-end multipath driver with properties similar to a basic multipath driver. A
path table is constructed with all back-end paths available for each LUN. The status of each path
and whether the path is active/active optimized or passive/active non-optimized is based on
information provided by the back-end storage device. There is no active load balancing and
back-end paths to each LUN are chosen at initialization time using a round robin approach between
available active/active optimized paths, so all paths are used equally. This approach does not
take into account the actual load placed on the LUNs during runtime. Changing paths to a LUN
only occurs in the event that the current back-end path being used is no longer available. Passive
and active non-optimized back-end paths are only used in the event that no active/active optimized
back-end paths to a LUN are available.
While the DPM back-end multipath driver is capable of detecting path types and doing simple
static load balancing, the limitations of the driver and how it can affect zoning should be considered.
The key limitation is that there is currently no mechanism in the DPM to control the back-end multipath
choices and the DPM makes these choices arbitrarily without regard for the I/O access patterns
to each LUN. By treating all LUNs equally in the round robin scheme, this may result in some cases
where the actual I/O load balance over the DPM back-end ports is inefficient. For example, if
there are 2 available active optimized paths to a back-end storage device with 4 LUNs presented
to the DPM, the DPM would access 2 LUNs along each active path. This approach would be
sufficient unless there are 2 LUNs that expect a large amount of I/O while the other 2 have only
a small amount of I/O . If the 2 LUNs with more I/O are accessed along the same fixed path, this
can lead to a situation where one back-end path has reached its peak capacity while the other is
underutilized.
In the case where situations such as the one described in the previous example are possible, greater
control over the available paths to each LUN may be required. This can be done by using a
combination of more restrictive zoning and LUN presentation to the DPM from the back-end storage
device. One possible approach that has been used is to zone each DPM back-end port to a storage
device port and presenting each LUN only to a specific DPM back-end port zoned to the managing
storage device controller for the LUN. This approach offers the most control over which back-end
port is used to access a given LUN but ignores any need for high availability that a customer
environment would have normally. A similar approach is to limit LUN access on a per-quad basis
where LUNs with more I/O can be separated onto different quads. The tradeoffs between
performance, high availability, and the complexity of the system should be considered if a custom
zoning and presentation configuration is implemented on the back-end.
DPM-VSM zoning
A DPM-VSM back-end zone is used to give the VSM access to the back-end initiator ports of the
DPM to manage the DPM LUN mapping information and facilitate data mover functions involving
mirroring, local snapshots, and remote replication. A back-end path between the DPM and the
VSM consists of a single DPM back-end initiator port and a single port on the VSM. In this type of
zone, the VSM acts in the target role. Each DPM back-end initiator port is zoned to all VSM ports
on the given fabric. Unlike the DPM-Host or DPM-Storage zoning, there is no limitation on the
number of paths that exist between the VSM and DPM. Each DPM-VSM zone has a single DPM
back-end initiator port and a single VSM port resulting in eight separate DPM-VSM zones for each
DPM quad.
Figure 16 (page 41) illustrates zoning between VSMs and the first quad of each DPM. This can
be duplicated for additional DPM quads by adding more zones. Paths between DPM initiator ports
and VSM target ports are color and shape coordinated to represent the different zones to which
they belong within the red and blue fabrics.
40 Zoning