Information Technology Reference
In-Depth Information
This distinction between array types is important because VMware's dei nition is based
on the multipathing mechanics, not whether you can use both storage processors at once. The
active-active and active-passive dei nitions apply equally to Fibre Channel (and FCoE) and iSCSI
arrays, and the virtual port dei nition applies to only iSCSI (because it uses an iSCSI redirection
mechanism that is not possible on Fibre Channel/FCoE).
The Fine Line between Active-Active and Active-Passive
Wondering why VMware specifi es “without signifi cant performance degradation” in the active-
active defi nition? h e reason is found within ALUA, a standard supported by many midrange arrays.
vSphere supports ALUA with arrays that implement ALUA compliant with the SPC-3 standard.
Midrange arrays usually have an internal interconnect between the two storage processors used
for write cache mirroring and other management purposes. ALUA was an addition to the SCSI
standard that enables a LUN to be presented on its primary path and on an asymmetrical (signifi -
cantly slower) path via the secondary storage processor, transferring the data over this internal
interconnect.
h e key is that the “non-optimized path” generally comes with a signifi cant performance degrada-
tion. h e midrange arrays don't have the internal interconnection bandwidth to deliver the same
response on both storage processors because there is usually a relatively small, or higher-latency,
internal interconnect used for cache mirroring that is used for A LUA versus enter prise arrays that
have a very-high-bandwidth internal model.
Without ALUA, on an array with an active-passive LUN ownership model, paths to a LUN are
shown as active, standby (designates that the port is reachable but is on a processor that does not
have the LUN), and dead. When the failover mode is set to ALUA, a new state is possible: active
non-optimized. h is is not shown distinctly in the vSphere Web Client GUI, but it looks instead
like a normal active path. h e diff erence is that it is not used for any I/O.
So, should you confi gure your midrange array to use ALUA? Follow your storage vendor's best
practice. For some arrays this is more important than others. Remember, however, that the non-
optimized paths will not be used (by default) even if you select the Round Robin policy. An active-
passive array using ALUA is not functionally equivalent to an active-passive array where all paths
are used. h is behavior can be diff erent if using a third-party multipathing module—see the section
“Reviewing Multipathing” later in this chapter.
By dei nition, all enterprise arrays are active-active arrays (by VMware's dei nition), but not
all midrange arrays are active-passive. To make things even more confusing, not all active-active
arrays (again, by VMware's dei nition) are enterprise arrays!
So, what do you do? What kind of array architecture is the right one for VMware? The
answer is simple: All of them on VMware's Hardware Compatibility List (HCL) work; you just
need to understand how the one
you
have works.
Most customers' needs are well met by midrange arrays, regardless of whether they have
an active-active, active-passive, or virtual port (iSCSI-only) design or whether they are NAS
devices. Generally, only the most mission-critical virtual workloads at the highest scale require
the characteristics of enterprise-class storage arrays. In these cases,
scale
refers to VMs that
