Introduction

	NOTE: You may need an additional license to use this feature.

The Dynamic Multipathing (DMP) feature of VERITAS Volume Manager (VxVM) provides greater reliability and performance by using path failover and load balancing. This feature is available for multiported disk arrays from various vendors. (See the VERITAS Volume Manager Hardware Notes for information about supported disk arrays.)

Multiported disk arrays can be connected to host systems through multiple paths. To detect the various paths to a disk, DMP uses a mechanism that is specific to each supported array type. DMP can also differentiate between different enclosures of a supported array type that are connected to the same host system.

The multipathing policy used by DMP depends on the characteristics of the disk array:

Active/passive disk arrays allow only one path (or primary path) to be used for I/O at a time. DMP only uses an alternate path (or secondary path) in the event that the primary path fails.

VxVM uses DMP metanodes to access disk devices connected to the system. For each disk in a supported array, DMP maps one metanode to the set of paths that are connected to the disk. Additionally, DMP associates the appropriate multipathing policy for the disk array with the metanode. For disks in an unsupported array, DMP maps a separate metanode to each path that is connected to a disk.

See Figure 3-1 “How DMP Represents Multiple Physical Paths to a Disk as one Metanode” for an illustration of how DMP sets up a metanode for a disk in a supported disk array.

Figure 3-1 How DMP Represents Multiple Physical Paths to a Disk as one Metanode

As described in “Enclosure-Based Naming”, VxVM 3.2 introduced an alternate disk device naming scheme that allows you to easily recognize to which array a disk belongs. Figure 3-2 “Example of Multipathing for a Disk Enclosure in a SAN Environment” shows that two paths, c1t99d0 and c2t99d0, exist to a single disk in the enclosure, but VxVM uses the single DMP metanode, enc0_0, to access it.

Figure 3-2 Example of Multipathing for a Disk Enclosure in a SAN Environment

See “Changing the Disk-Naming Scheme” for details of how to change the naming scheme that VxVM uses for disk devices.

	NOTE: The operation of DMP relies on the vxdmp device driver. Unlike prior releases, from VxVM 3.1.1 onwards, the vxdmp driver must always be present on the system.

Path Failover Mechanism

The DMP feature of VxVM enhances system reliability when used with multiported disk arrays. In the event of the loss of one connection to the disk array, DMP automatically selects the next available I/O path for I/O requests dynamically without action from the administrator.

DMP is also informed when you repair or restore a connection, and when you add or remove devices after the system has been fully booted (provided that the operating system recognizes the devices correctly).

Load Balancing

DMP uses the balanced path mechanism to provide load balancing across paths for active/active disk arrays. Load balancing maximizes I/O throughput by using the total bandwidth of all available paths. Sequential I/O starting within a certain range is sent down the same path in order to benefit from disk track caching. Large sequential I/O that does not fall within the range is distributed across the available paths to reduce the overhead on any one path.

For active/passive disk arrays, I/O is sent down the primary path. If the primary path fails, I/O is switched over to the other available primary paths or secondary paths. As the continuous transfer of ownership of LUNs from one controller to another results in severe I/O slowdown, load balancing across paths is not performed for active/passive disk arrays.