IBM System Storage DS4000, Руководство по оборудованию

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Chapter 4. IBM System Storage DS planning and configuration
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One path per controller: Failover
Failover is not a balancing algorithm, but we include it here because it is an option that is
presented with the actual balancing policies in some operating systems, such as Windows.
The driver use failover only when there is one path to each controller of your system. Using
failover, one of the device paths is active, the one corresponding to the controller who owns
the logical volume, and the other is in a standby state. This is the default for RDAC drivers
(AIX and Solaris), because it only supports two paths, that is, one to each controller. Make
sure to zone accordingly when working with RDAC so that there is not more than one path per
controller.
Multiple paths per controller
The multi-path driver transparently balances I/O workload without administrator intervention,
across multiple paths to the same controller, but not across both controllers. To make use of
the feature, you need to cable and zone your SAN properly. The load balancing policy uses
one of three following algorithms:
򐂰
Round robin with subset
The round robin with subset I/O load balance policy routes I/O requests, in rotation, to
each available data path to the controller that owns the volumes. This policy treats all
paths to the controller that owns the volume equally for I/O activity. Paths to the secondary
controller are ignored until ownership changes. The basic assumption for the round robin
policy is that the data paths are equal. With mixed host support, the data paths might have
different bandwidths or different data transfer speeds.
򐂰 Least queue depth with subset
The least queue depth with subset policy is also known as the least I/Os or least requests
policy. This policy routes the next I/O request to a data path that has the least outstanding
I/O requests queued. For this policy, an I/O request is simply a command in the queue.
The type of command or the number of blocks that are associated with the command are
not considered. The least queue depth with subset policy treats large block requests and
small block requests equally. The data path selected is one of the paths in the path group
of the controller that owns the volume.
򐂰 Least path weight with subset
The least path weight with subset policy assigns a weight factor to each data path to a
volume. An I/O request is routed to the path with the lowest weight value to the controller
that owns the volume. If more than one data path to the volume has the same weight
value, the round-robin with subset path selection policy is used to route I/ O requests
between the paths with the same weight value.
4.4.5 Microsoft Windows MPIO
This section discusses the available Windows multipath options.
MPIO is a Driver Development Kit (DDK) from Microsoft for developing code that manages
multipath devices. It contains a core set of binary drivers, which are installed with the DS5000
Device Specific Module (DSM) to provide a transparent system architecture that relies on
Microsoft Plug and Play to provide LUN multipath functionality while maintaining compatibility
with existing Microsoft Windows device driver stacks.
Note: The MPIO Driver is included in the Storage Manager software package for Windows
and supports Microsoft Windows 2003 and 2008 on 32-bit and x64 systems. In Windows
2008, MPIO is already part of the operating system.