has dominated the storage area network market
for several years now, and its cost and
complexity are finally becoming apparent.
Meanwhile, IP SANs--particularly the
long-awaited iSCSI SAN--have emerged and
warrant your consideration.
Oct 2, 2003
By Steven Schuchart Jr.
much to offer. The most obvious benefit is
cost. Ethernet technology is significantly
less expensive than Fibre Channel. Fibre
Channel ports cost as much as 50 percent more
than copper Gigabit Ethernet ports, and that
doesn't include the cost of the optics.
most IT administrators are familiar with
deployment and provisioning of Ethernet and
TCP/IP networks. In fact, virtually every
college in the country teaches Ethernet and
TCP/IP technologies. With all that knowledge
already in-house, your initial SAN
implementation would be easier. Fibre Channel,
on the other hand, is complicated--it demands
training, which can drain your time, energy
and financial resources.
advantage of iSCSI compared with Fibre Channel
is the distance over which it can be used.
Fibre Channel is limited in this capacity, but
iSCSI uses TCP/IP, which means it can be set
up for use at great distance over the Internet
for very light storage traffic or over leased
lines without the need for the expensive
optical gear Fibre Channel requires.
Need for Speed
have some drawbacks. Current iSCSI technology
is limited to 1 Gbps--the speed of Gigabit
Ethernet--while Fibre Channel solutions have a
maximum transfer rate of 2 Gbps (4-Gigabit FC
is on the immediate horizon). We expect that
iSCSI eventually will be deployed on 10
Gigabit Ethernet, but for now, that's not
possible--there isn't a TCP/IP off-load engine
capable of handling those speeds.
In three to
five years, when 10 Gigabit Ethernet becomes
more prevalent, you'll see iSCSI follow suit.
But if your current requirements for
responsiveness and backup speed can't be met
at 1 Gbps, iSCSI is not the technology for you
you've already invested a great deal in Fibre
Channel technology, you may want to think
seriously about making a considerable iSCSI
investment. There is a definite case for using
iSCSI in a limited capacity for distance
applications and in less speed-critical
situations. It can also be an inexpensive way
to move forward if you are currently at
capacity on your Fibre Channel switches.
this: You shouldn't expect to run your SAN
network on the same generalized TCP/IP network
that carries data between servers and users.
Storage is simply too important to be on the
generalized data network--it must be
segregated onto separate VLANs or an entirely
separate switched network. (There are
exceptions to that rule, as in the case of a
small workgroup using iSCSI for limited access
to a main storage device or accessing storage
consideration: iSCSI places a large burden on
a server's CPU. iSCSI is essentially a wrapper
around standard SCSI commands. Those commands
must be received in order, and each command or
data packet must be wrapped (or unwrapped).
This creates a huge amount of TCP/IP work that
the standard NIC card passes off to the CPU.
is a TCP Off-load Engine (TOE) or an iSCSI
Host Bus Adapter (HBA). These specialized
cards perform all the TCP/IP overhead
processing with on-board processors instead of
passing it to the CPU the way standard NIC
TOE cards and
iSCSI HBAs each have advantages and drawbacks.
Whereas iSCSI HBAs deal only with iSCSI
traffic, TOE cards can handle all standard
Ethernet traffic. However, iSCSI HBAs use
fewer main CPUs than do TOE cards for iSCSI
operations, and they generally cost a few
hundred dollars less than TOEs.
In our test
of TOE cards and iSCSI HBAs, (see "Don't
Sink Your IP SAN" at www.nwc.com/1409/
1409f3), we found that if a TOE card or HBA
has only one target, it's unlikely to achieve
a data rate of 100 Mbps because parallelism in
the off-load processors prevent it.
in the TOE-versus-iSCSI-card debate include
whether you intend to repurpose the card and
whether the CPU on the server could benefit
from the off-loading of other non-iSCSI TCP/IP
iSCSI drawbacks make you think twice, remember
that Fibre Channel still remains the dominant
SAN technology, and FC vendors have made great
strides in their battle against almost
legendary compatibility problems (see
"High on Fibre" at www.nwc.com/1325/1325f5.html).
In addition, much progress has been made in
managing Fibre Channel hardware, though there
is still much to do. For example, software
tools that manage hardware from multiple
vendors can only handle about 50 percent of
the hardware out there. Interfaces and
terminology from vendor to vendor still vary
widely, especially regarding the names of
specific kinds of ports. Bottom line: The
Fibre Channel industry can't yet manage the
number the ports on the outside of the chassis
in the same manner.
decided to bring iSCSI to your SAN, what's the
best way to proceed? Start with the storage
itself. All the usual considerations for a
storage array come into play: reliability,
speed, capacity, expandability and price.
consider what type of hard disk drive you'll
need. There's no such thing as an Ethernet
hard disk drive. All drives are a form of
SCSI, Fibre Channel or ATA (AT Attachment).
Thankfully, some companies, such as IBM, LSI
Logic and Adaptec's Eurologic Systems, make
enclosures that act as iSCSI targets--this
eliminates the need to buy a translative
device. If you buy the storage device with its
native interface, however, you must attach it
to a storage switch from Cisco Systems, McData
(Nishan Systems), Crossroads Networks or
another translative device vendor, which
creates an extra step. Still, if you already
have these drives, that's the way to go--use
the brand of Ethernet switch you're used to.
If you're a Cisco shop, use Cisco. If you're a
Nortel shop, use Nortel.
switch, an iSCSI packet is just another TCP/IP
packet. Network usage levels are determined
the same way as they are on the rest of your
Ethernet network. At the server or, in iSCSI
terms, the initiator, you simply have to
choose a TOE card or an iSCSI HBA. iSCSI HBAs
generally come with their own software
initiator, while TOE cards use a third-party
initiator to work. Microsoft and the
open-source community offer iSCSI initiators