Information Technology Reference
In-Depth Information
Understanding Midrange and External Enterprise Storage Array
Design
There are some major differences in physical array design that can be pertinent in a vSphere
design.
Traditional external midrange storage arrays are generally arrays with dual-storage processor
cache designs where the cache is localized to one storage processor or another but commonly
mirrored between them. (Remember that all vendors call storage processors something slightly
different; sometimes they are called
controllers
,
heads
,
engines
, or
nodes
.) In cases where one of the
storage processors fails, the array remains available, but in general, performance is degraded
(unless you drive the storage processors to only 50 percent storage processor utilization during
normal operation).
External enterprise storage arrays are generally considered to be those that scale to many
more controllers and a much larger global cache (memory can be accessed through some com-
mon shared model). In these cases, multiple elements can fail while the array is being used at a
very high degree of utilization—without any signii cant performance degradation. Enterprise
arrays can also include support for mainframes, and there are other characteristics that are
beyond the scope of this topic.
Hybrid designs exist as well, such as scale-out designs where they can scale out to more than
two storage processors but without the features otherwise associated with enterprise storage
arrays. Often these are iSCSI-only arrays and leverage iSCSI redirection techniques (which are
not options of the Fibre Channel or NAS protocol stacks) as a core part of their scale-out design.
Design can be confusing, however, because VMware and storage vendors use the same
words to express different things. To most storage vendors, an
active
-
active
storage array is an
array that can service I/O on all storage processor units at once, and an
active
-
passive
design is
a system where one storage processor is idle until it takes over for the failed unit. VMware has
specii c nomenclature for these terms that is focused on the model for a
specii c
LUN
. VMware
dei nes active-active and active-passive arrays in the following way (this information is taken
from the
vSphere
Storage
Guide
):
Active-Active Storage System
An active-active storage system provides access to LUNs
simultaneously through all available storage ports without signii cant performance degrada-
tion. Barring a path failure, all paths are active at all times.
Active-Passive Storage System
In an active-passive storage system, one storage processor
is actively providing access to a given LUN. Other processors act as backup for the LUN and
can be actively servicing I/O to other LUNs. In the event of the failure of an active storage
port, one of the passive storage processors can be activated to handle I/O.
Asymmetrical Storage System
An asymmetrical storage system supports Asymmetric
Logical Unit Access (ALUA), which allows storage systems to provide different levels of
access per port. This permits the hosts to determine the states of target ports and establish
priority for paths. (See the sidebar “The Fine Line between Active-Active and Active-Passive”
for more details on ALUA.)
Virtual Port Storage System
Access to all LUNs is provided through a single virtual port.
These are active-active devices where the multiple connections are disguised behind the
single virtual port. Virtual port storage systems handle failover and connection balancing
transparently, which is often referred to as “transparent failover.”
