Dell PowerVault ME4 Series Storage System Administrator’s Guide

About volume mapping

Mappings between a volume and one or more initiators, hosts, or host groups enable hosts to view and access the volume. There are two types of maps that can be created: default maps and explicit maps. Default maps enable all hosts to see the volume using a specified LUN and access permissions. Default mapping applies to any host that has not been explicitly mapped using different settings.

The advantage of a default mapping is that all connected hosts can discover the volume with no additional work by the administrator. The disadvantage is that all connected hosts can discover the volume with no restrictions. Therefore, this process is not recommended for specialized volumes that require restricted access.

When a volume is created, it is not mapped by default. You can create default or explicit mappings for it. You can change the default mapping of a volume, and create, modify, or delete explicit mappings. A mapping can specify read-write, read-only, or no access through one or more controller host ports to a volume. When a mapping specifies no access, the volume is masked.

For example, a payroll volume could be mapped with read-write access for the Human Resources host and be masked for all other hosts. An engineering volume could be mapped with read-write access for the Engineering host and read-only access for other departments' hosts.

A LUN identifies a mapped volume to a host. Both controllers share a set of LUNs, and any unused LUN can be assigned to a mapping. However, each LUN is generally only used once as a default LUN. For example, if LUN 5 is the default for Volume1, no other volume in the storage system can use LUN 5 on the same port as its default LUN. For explicit mappings, the rules differ: LUNs used in default mappings can be reused in explicit mappings for other volumes and other hosts.

The storage system uses Unified LUN Presentation (ULP), which can expose all LUNs through all host ports on both controllers. The interconnect information is managed in the controller firmware. ULP appears to the host as an active-active storage system where the host can choose any available path to access a LUN regardless of disk group ownership. When ULP is in use, the controllers' operating redundancy mode is shown as Active-Active ULP. ULP uses the INCITS T10 Technical Committee Asymmetric Logical Unit Access (ALUA) extensions, in SPC-3, to negotiate paths with aware host systems. Unaware host systems see all paths as being equal.