Dell PowerVault ME4 Series Storage System Administrator’s Guide

Mapping initiators and volumes

You can map initiators and volumes to control host access to volumes unless the volume is the secondary volume of a replication set. Mapping applies to hosts and host groups as well as initiators, and to virtual snapshots and volume groups as well as volumes. For the purposes of brevity, the terms initiator and volumes will stand in for all possibilities, unless otherwise stated. By default, volumes are not mapped.

If a volume is mapped to ID All Other Initiators, this is its default mapping. The default mapping enables all connected initiators to see the volume using the specified access mode, LUN, and port settings. The advantage of a default mapping is that all connected initiators can discover the volume with no additional work by the administrator. The disadvantage is that all connected initiators can discover the volume with no restrictions. Therefore, this process is not recommended for specialized volumes that require restricted access. Also, to avoid multiple hosts mounting the volume and causing corruption, the hosts must be cooperatively managed, such as by using cluster software.

If multiple hosts mount a volume without being cooperatively managed, volume data is at risk for corruption. To control access by specific hosts, you can create an explicit mapping. An explicit mapping can use different access mode, LUN, and port settings to allow or prevent access by a host to a volume, overriding the default mapping. When an explicit mapping is deleted, the volume's default mapping takes effect.

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 T10 Technical Committee of INCITS 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.

If a host group or host is mapped to a volume or volume group, all of the initiators within that group will have an individual map to each volume that makes up the request. As long as the group entity is mapped consistently, that set of individual maps will be represented as a grouped mapping. If any individual map within that group is modified, the grouped mapping will no longer be consistent, and it will no longer appear in the PowerVault Manager. It will be replaced in the PowerVault Manager with all of the individual maps.