HP StorageWorks Enterprise Virtual Array Cluster FIO Starter Kit Uživatelská příručka stáhnout pdf (Strana 42)

the active non-optimized paths for reads to a LUN results in internal proxy I/Os between storage

controllers and should be avoided.

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 14 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.

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