Although
iSCSI technology enables cost-effective
block-based storage networking, its deployment has generally
been restricted to SMB/SME and departmental workgroup users --
mainly due to the limitations inherent with
Ethernet networks such as IP overhead, latency, port
oversubscription and iSCSI target initiator software. So, while an
iSCSI
storage area network (SAN) "could" be
assembled with common and inexpensive Ethernet components, it
probably wouldn't work very well under real operating
conditions. In order for iSCSI to compete and flourish in the
enterprise, storage and network administrators must address
these traditional problems.
One part of the solution is improved Ethernet hardware,
including Ethernet network interface cards (NICs) that incorporate
a TCP/IP Offload Engine (TOE). TOE chips offload some or all of the
work required to process TCP/IP data, reducing workload on the
local CPU and improving Ethernet efficiency. TOE cards have not
been very popular because of their added expense, but their
deployment is expanding as iSCSI SAN traffic rates increase. TOE
NICs are used most often in the storage system (target) where the
I/O traffic from multiple servers (initiators) will come together
and cause congestion. TOE cards are available from just a handful
of vendors, including Alacritech, LeWiz Communications Inc., and
QLogic Corp.
Common Ethernet switch ports tend to introduce latency into
iSCSI traffic, and this reduces performance. Experts suggest
deploying high-performance Ethernet switches that sport fast,
low-latency ports. In addition, you may choose to tweak iSCSI
performance further by overriding "auto-negotiation" and manually
adjusting speed settings on the NIC and switch. This lets you
enable traffic flow control on the NIC and switch, setting Ethernet
jumbo frames on the NIC and switch to 9000 bytes or higher --
transferring far more data in each packet while requiring less
overhead. Jumbo frames are reported to improve throughput as much
as 50%. High-performance IP storage switches/routers are available
from Brocade Communications Systems Inc., Cisco Systems Inc.,
Emulex Corp., Hewlett-Packard Co., SANRAD Inc., and Woven Systems
Inc.
Switch port performance can also be enhanced by eliminating
"oversubscription." There's no harm in oversubscription when
bandwidth is underutilized -- multiple devices can help to fill
underutilized bandwidth and achieve more value per port. But as
utilization goes up, devices can begin competing for the port,
introducing latency and performance problems for an iSCSI SAN.
Rather than allowing multiple devices to compete for one switch
port, establish a limit of one device per port.
It's important to consider the performance of your iSCSI
initiator (server-side) software. As with any device driver, the
quality and integrity of your iSCSI initiator software can vary
dramatically depending on the vendor, their experience in the iSCSI
market, and the maturity of their iSCSI product -- some initiators
simply work better than others. It may be worthwhile to test the
performance and robustness of several iSCSI initiators before
deciding on the best initiator. TOE cards and other hardware
devices include their own initiator firmware, eliminating the need
for separate initiator software.
Finally, iSCSI SAN performance can be improved through careful
logical and physical separation. For example, iSCSI SAN traffic
should never be mixed with ordinary Ethernet user traffic. This not
only impairs SAN performance, but also creates a potential security
risk since storage data is accessible on the user LAN. Instead,
iSCSI SAN traffic should be isolated from the everyday user
traffic. The most common means of separation is a virtual LAN
(VLAN), limiting iSCSI traffic to the virtual LAN and keeping
regular traffic out. The same effect can be achieved physically by
creating a new LAN segment for the iSCSI SAN (often using high
performance NIC and switch hardware), and keeping that segment
isolated from other regular Ethernet segments.
Check out the entire
iSCSI vs. FC handbook.