Information Technology Reference
In-Depth Information
has a minimum of two connections to two redundant front-end array network interfaces (across
storage processors).
The one additional concept to focus on with iSCSI is the concept of
fan
-
in
ratio
. This applies
to all shared storage networks, including Fibre Channel, but the effect is often most pronounced
with Gigabit Ethernet (GbE) networks. Across all shared networks, there is almost always a
higher amount of bandwidth available across all the host nodes than there is on the egress of the
switches and front-end connectivity of the array. It's important to remember that the host band-
width is gated by congestion wherever it occurs. Don't minimize the array port-to-switch con-
i guration. If you connect only four GbE interfaces on your array and you have 100 hosts with
two GbE interfaces each, then expect contention, because your fan-in ratio is too large.
Also, when iSCSI and iSCSI SANs are examined, many core ideas are similar to Fibre
Channel and Fibre Channel SANs, but in some cases there are material differences. Let's look at
the terminology:
iSCSI Initiator
An iSCSI initiator is a logical host-side device that serves the same function
as a physical host bus adapter in Fibre Channel/FCoE or SCSI/SAS. iSCSI initiators can be
software initiators (which use host CPU cycles to load/unload SCSI payloads into standard
TCP/IP packets and perform error checking) or hardware initiators (the iSCSI equivalent of
a Fibre Channel HBA or FCoE CNA). Examples of software initiators that are pertinent to
vSphere administrators are the native ESXi software initiator and the guest software initia-
tors available in Windows XP and later and in most current Linux distributions. Examples
of iSCSI hardware initiators are add-in cards like the QLogic QLA 405x and QLE 406x host
bus adapters. These cards perform all the iSCSI functions in hardware. An iSCSI initiator is
identii ed by an iSCSI qualii ed name (referred to as an IQN). An iSCSI initiator uses an iSCSI
network portal that consists of one or more IP addresses. An iSCSI initiator “logs in” to an
iSCSI target.
iSCSI Target
An iSCSI target is a logical target-side device that serves the same function as
a target in Fibre Channel SANs. It is the device that hosts iSCSI LUNs and masks to specii c
iSCSI initiators. Different arrays use iSCSI targets differently—some use hardware, some use
software implementations—but largely this is unimportant. More important is that an iSCSI
target doesn't necessarily map to a physical port as is the case with Fibre Channel; each array
does this differently. Some have one iSCSI target per physical Ethernet port; some have one
iSCSI target per iSCSI LUN, which is visible across multiple physical ports; and some have
logical iSCSI targets that map to physical ports and LUNs in any relationship the administra-
tor coni gures within the array. An iSCSI target is identii ed by an iSCSI qualii ed name (an
IQN). An iSCSI target uses an iSCSI network portal that consists of one or more IP addresses.
iSCSI Logical Unit
An iSCSI LUN is a LUN hosted by an iSCSI target. There can be one or
more LUNs behind a single iSCSI target.
iSCSI Network Portal
An iSCSI network portal is one or more IP addresses that are used
by an iSCSI initiator or iSCSI target.
iSCSI Qualii ed Name
An iSCSI qualii ed name (IQN) serves the purpose of the WWN in
Fibre Channel SANs; it is the unique identii er for an iSCSI initiator, target, or LUN. The for-
mat of the IQN is based on the iSCSI IETF standard.
Challenge Authentication Protocol
CHAP is a widely used basic authentication protocol,
where a password exchange is used to authenticate the source or target of communication.
