Storage Foundation 8.0 Administrator's Guide - Windows

These modes also apply to the array's disks and are defined as follows:

• Active/Active

  The mode in which Dynamic Multi-Pathing allocates the data transfer across all the possible paths, thus enabling the desirable feature of load balancing. With this mode, Dynamic Multi-Pathing implements a round-robin algorithm, selecting each path in sequence for each successive data transfer to or from a disk. For example, if you have two paths active, A and B, the first disk transfer occurs on path A, the next on path B, and the next on path A again.

  In addition to the round-robin algorithm, DMP DSMs offer the following load balancing options:

  • Dynamic Least Queue Depth

    Selects the path with the least number of I/O requests in its queue for the next data transfer.

    For example, if you have two active paths, path A with one I/O request and path B with none, DMP DSMs selects the path with the least number of I/O requests in its queue, path B, for the next data transfer.

  • Balanced Path

    This policy is designed to optimize the use of caching in disk drives and RAID controllers. The size of the cache depends on the characteristics of the particular hardware. Generally, disks and LUNs are logically divided into a number of regions or partitions. I/O to and from a given region is sent on only one of the active paths. Adjusting the region size to be compatible with the size of the cache is beneficial so that all the contiguous blocks of I/O to that region use the same active path. The value of the partition size can be changed by adjusting the value of the tunable parameter, Block Shift.

    Block Shift represents the number of contiguous I/O blocks that are sent along a path to an Active/Active array before switching to the next available path. The Block Shift value is expressed as the integer exponent of a power of 2. For example, the Block Shift value of 11 represents 211 or 2048 contiguous blocks of I/O.

    The benefit of this policy is lost if the value is set larger than the cache size. The benefit is also lost when the active path fails. In this situation, the I/O is automatically redistributed across the remaining paths.

    The default value of the Block Shift parameter is set to 11 so that 2048 blocks (1MB) of contiguous I/O are sent over a path before switching to a different path. Depending on your hardware, adjusting this parameter may result in better I/O throughput. Refer to your hardware documentation for more information.

    Note:

    Block Shift only affects the behavior of the balanced path policy. A value of 0 disables multi-pathing for the policy unless the vxdmpadm command is used to specify a different partition size for an array.

  • Weighted Paths

    Uses the path with the lowest numerical weight. Each path is assigned a weight by the user to designate which path is favored for data transfer. If two or more paths have the same weight and are the lowest weight of all paths, then these paths are used each in turn, in round-robin fashion, for the data transfer.

    For example, if you have three active paths, path A with weight of 0, path B with weight of 0, and path C with weight of 9, DMP DSMs use path A for one data transfer and then use path B for the next. Path C is in standby mode and is used if path A or path B fails.

  • Round robin with Subset

    Uses a subset of paths, each in turn, in round-robin fashion. The user specifies the paths for data transfer that make up the subset. The remaining paths are in standby mode.

    For example, if you have three active paths, path A, path B, and path C and you specify the subset to contain path A and path B, then DMP DSMs use path A for one data transfer and then use path B for the next. Path C is in standby mode and is used if path A or path B fails.

  • Least Blocks

    Selects the path with the least number of blocks of I/O in its queue for the next data transfer.

    For example, if you have two active paths, path A with one block of I/O and path B with none, DMP DSMs select the path with the least number of blocks of I/O in its queue, path B, for the next data transfer.

• Active/Passive

  A mode in which a path that is designated as the "Preferred Path" or "primary path " is always active and the other path or paths act as backups (standby paths) that are called into service if the current operating path fails.

• All paths to the disks are current active I/O paths. Each active path is designated by a path icon with a green circle in the VEA GUI.

• For an Active/Passive load balance setting, the primary path is designated by a path icon with a checkmark in a green circle in the GUI.

• The DMP DSMs are not enabled to indicate which array controller each path is connected to.

• The current active path is designated by a path icon with a green circle in the VEA GUI.

• For an Active/Passive load balance setting, the primary path is designated by a path icon with a checkmark in a green circle in the VEA GUI.

• DMP automatically selects the primary path for Active/Passive load balancing.

• Round robin with Subset and Weighted Paths load balance settings are available only at the device level. They are not available at the array level.

• Active paths are connected to the same array controller.