vSphere Storage - ESXi 5.1 - Documentation - VMware

vSphere Storage
Figure 10-1. Target Compared to LUN Representations
target
LUN LUN LUN
storage array
target target target
LUN LUN LUN
storage array
Three LUNs are available in each of these configurations. In the first case, the host detects one target but that
target has three LUNs that can be used. Each of the LUNs represents individual storage volume. In the second
case, the host detects three different targets, each having one LUN.
Host-based iSCSI initiators establish connections to each target. Storage systems with a single target containing
multiple LUNs have traffic to all the LUNs on a single connection. With a system that has three targets with
one LUN each, a host uses separate connections to the three LUNs. This information is useful when you are
trying to aggregate storage traffic on multiple connections from the host with multiple iSCSI HBAs, where
traffic for one target can be set to a particular HBA, while traffic for another target can use a different HBA.
iSCSI Storage System Types
ESXi supports different storage systems and arrays.
The types of storage that your host supports include active-active, active-passive, and ALUA-compliant.
Active-active storage
system
Active-passive storage
system
Asymmetrical storage
system
Virtual port storage
system
Discovery, Authentication, and Access Control
Allows access to the LUNs simultaneously through all the storage ports that
are available without significant performance degradation. All the paths are
active at all times, unless a path fails.
A system in which one storage processor is actively providing access to a given
LUN. The other processors act as backup for the LUN and can be actively
providing access to other LUN I/O. I/O can be successfully sent only to an active
port for a given LUN. If access through the active storage port fails, one of the
passive storage processors can be activated by the servers accessing it.
Supports Asymmetric Logical Unit Access (ALUA). ALUA-complaint storage
systems provide different levels of access per port. ALUA allows hosts to
determine the states of target ports and prioritize paths. The host uses some of
the active paths as primary while others as secondary.
Allows access to all available LUNs through a single virtual port. These are
active-active storage devices, but hide their multiple connections though a
single port. ESXi multipathing does not make multiple connections from a
specific port to the storage by default. Some storage vendors supply session
managers to establish and manage multiple connections to their storage. These
storage systems handle port failover and connection balancing transparently.
This is often referred to as transparent failover.
You can use several mechanisms to discover your storage and to limit access to it.
You must configure your host and the iSCSI storage system to support your storage access control policy.
74 VMware, Inc.