Administrator Guide

CHAP definitions. This information may be useful in configuring CHAP entries for new hosts. This information becomes visible when an
iSCSI discovery session is established, because the storage system does not require discovery sessions to be authenticated. CHAP
authentication must succeed for normal sessions to move to the full feature phase.
About volume mapping
Mappings between a volume and one or more initiators, hosts, or host groups hosts 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.
Explicit maps override a volume's default map for specific hosts.
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.
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 a different access mode, LUN, and port settings to allow or prevent
access by a host to a volume. If there is a default mapping, the explicit mapping overrides it.
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.
NOTE:
When an explicit mapping is deleted, the volume's default mapping takes effect. Though default mappings can be
used for specific installations, using explicit mappings with hosts and host groups is recommended for most
installations.
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.
About operating with a single controller
If you purchased a 2U controller enclosure with a single controller module, note that it does not offer redundant configuration and, in the
case of controller failure, leaves the system at risk for data unavailability. For more information, see About data protection with a single
controller.
NOTE:
If you are operating a system with a single controller, some functionality described in the documentation may be
unavailable or not applicable to your system. For example, only one storage pool can exist and text about controller
failover and recovery is not applicable.
About snapshots
The system can create snapshots of virtual volumes up to the maximum number supported by your system. Snapshots provide data
protection by enabling you to create and save source volume data states at the point in time when the snapshot was created. Snapshots
can be created manually or you can schedule snapshot creation. After a snapshot has been created, the source volume cannot be
expanded.
When you reach the maximum number of snapshots for your system, before you can create a new snapshot, you must delete an existing
snapshot. To view the maximum number of snapshots for your system, see the System configuration limits topic in the PowerVault
Manager help.
Getting started
25