Search <book_title>...

Veritas InfoScale™ Virtualization Guide - Linux on ESXi

Last Published: 2019-02-26

Product(s): InfoScale & Storage Foundation (7.4)

Platform: VMware ESX

Section I. Overview
1. About Veritas InfoScale solutions in a VMware environment
Section II. Deploying Veritas InfoScale products in a VMware environment
1. Getting started
2. Understanding Storage Configuration
Section III. Use cases for Veritas InfoScale product components in a VMware environment
Section IV. Reference
1. Appendix A. Known issues and limitations
  1. Prevention of Storage vMotion
2. Appendix B. Where to find more information

How DMP works

Dynamic Multi-Pathing (DMP) provides greater availability, reliability, and performance by using the path failover feature and the load balancing feature. These features are available for multiported disk arrays from various vendors.

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. DMP can also differentiate between different enclosures of a supported array that are connected to the same host system.

The multi-pathing policy that DMP uses depends on the characteristics of the disk array.

DMP supports the following standard array types:

Table:

Array type	Description
Active/Active (A/A)	Allows several paths to be used concurrently for I/O. Such arrays allow DMP to provide greater I/O throughput by balancing the I/O load uniformly across the multiple paths to the LUNs. In the event that one path fails, DMP automatically routes I/O over the other available paths.
Asymmetric Active/Active (A/A-A)	A/A-A or Asymmetric Active/Active arrays can be accessed through secondary storage paths with little performance degradation. The behavior is similar to ALUA, except that it does not support the SCSI commands that an ALUA array supports.
Asymmetric Logical Unit Access (ALUA)	DMP supports all variants of ALUA.
Active/Passive (A/P)	Allows access to its LUNs (logical units; real disks or virtual disks created using hardware) via the primary (active) path on a single controller (also known as an access port or a storage processor) during normal operation. In implicit failover mode (or autotrespass mode), an A/P array automatically fails over by scheduling I/O to the secondary (passive) path on a separate controller if the primary path fails. This passive port is not used for I/O until the active port fails. In A/P arrays, path failover can occur for a single LUN if I/O fails on the primary path. This array mode supports concurrent I/O and load balancing by having multiple primary paths into a controller. This functionality is provided by a controller with multiple ports, or by the insertion of a SAN switch between an array and a controller. Failover to the secondary (passive) path occurs only if all the active primary paths fail.
Active/Passive in explicit failover mode or non-autotrespass mode (A/PF)	The appropriate command must be issued to the array to make the LUNs fail over to the secondary path. This array mode supports concurrent I/O and load balancing by having multiple primary paths into a controller. This functionality is provided by a controller with multiple ports, or by the insertion of a SAN switch between an array and a controller. Failover to the secondary (passive) path occurs only if all the active primary paths fail.
Active/Passive with LUN group failover (A/PG)	For Active/Passive arrays with LUN group failover (A/PG arrays), a group of LUNs that are connected through a controller is treated as a single failover entity. Unlike A/P arrays, failover occurs at the controller level, and not for individual LUNs. The primary controller and the secondary controller are each connected to a separate group of LUNs. If a single LUN in the primary controller's LUN group fails, all LUNs in that group fail over to the secondary controller. This array mode supports concurrent I/O and load balancing by having multiple primary paths into a controller. This functionality is provided by a controller with multiple ports, or by the insertion of a SAN switch between an array and a controller. Failover to the secondary (passive) path occurs only if all the active primary paths fail.

An array policy module (APM) may define array types to DMP in addition to the standard types for the arrays that it supports.

Veritas InfoScale uses DMP metanodes (DMP nodes) to access disk devices connected to the system. For each disk in a supported array, DMP maps one node to the set of paths that are connected to the disk. Additionally, DMP associates the appropriate multi-pathing policy for the disk array with the node.

Figure: How DMP represents multiple physical paths to a disk as one node shows how DMP sets up a node for a disk in a supported disk array.

Figure: How DMP represents multiple physical paths to a disk as one node

DMP implements a disk device naming scheme that allows you to recognize to which array a disk belongs.

Figure: Example of multi-pathing for a disk enclosure in a SAN environment shows an example where two paths, vmhba1:C0:T0:L0 and vmhba2:C0:T0:L0, exist to a single disk in the enclosure, but VxVM uses the single DMP node, enc0_0, to access it.

Figure: Example of multi-pathing for a disk enclosure in a SAN environment