Storage networks allow disparate storage resources to be
centralized and organized within the data center. The available
storage can then be centrally managed and allocated to users or
applications.
Fibre Channel (FC) has been the dominant
storage area network (SAN) architecture for
years, but its high cost and management complexity have deterred
many SMBs from adopting this SAN technology.
Internet SCSI, or
ISCSI, addresses these SAN problems by
carrying
SCSI commands and storage traffic over
existing
IP networks. Transporting "SCSI over IP"
leverages ubiquitous Ethernet networks, and that translates to
lower costs, easier maintenance and simpler management, which in
turn makes SAN deployment more attractive to SMBs. This article
examines the basics of iSCSI, enumerates deployment roadblocks
and anticipates future trends.
Understanding iSCSI SANs
The development of iSCSI is a good example of problem-solving by
using existing technologies in new ways. Traditional FC storage
networks use the SCSI command set, carrying SCSI storage commands
over physical FC infrastructures. As developers sought ways to
overcome the cost, configuration and management complexities
associated with FC networks, it made sense to place SCSI commands
across other network architectures. Thus, iSCSI is SCSI
block-storage access that is mapped to run across an IP network --
almost universally an Ethernet network. Although the iSCSI standard
was ratified in 2003, iSCSI technology has only recently started
receiving broad attention.
@31334 iSCSI SANs offers several benefits to an organization;
cost, labor/management, and reach. The ubiquitous nature of
Ethernet means that IP networks can be deployed quickly and easily
in organizations of all sizes. Ethernet is also readily understood,
so IT personnel can deploy and maintain an IP environment without
specialized FC SAN training. It's not necessary to hire more IT
personnel to implement and manage the SAN. Where FC SANs are
typically small islands of technology located in a data center, the
use of IP networks also gives iSCSI a global reach leading from a
LAN to a
WAN and onto the Internet -- allowing
storage to be located almost anywhere.
Although the terms "iSCSI" and "storage over IP" are often used
interchangeably, this is technically incorrect. While iSCSI may be
the dominant subset of IP storage, there are other IP storage
technologies to consider, such
asFCIP (Fibre Channel over IP) for exchanging
data between FC networks across the Internet and
iFCP (Internet Fibre Channel protocol) for
extending FC networks across the Internet.
Debunking common IP network myths
The low cost of iSCSI is usually its strongest selling point,
but the issue of iSCSI cost is a bit more complicated than people
think. People buy iSCSI because they want a cheaper alternative to
FC. "Users have $2,000 Windows servers that they'd love to put on a
SAN, but they can't afford to spend $2,000 more to connect them to
a Fibre Channel SAN," says Stephen Foskett, director of strategy
services at GlassHouse Technologies Inc. Leveraging an existing
Ethernet network can indeed be far less expensive than deploying
and interfacing to a new FC SAN, but businesses will ultimately
spend capital on iSCSI storage arrays or specialized iSCSI adapters
to enhance a server's connectivity.
The IP network itself is often a point of confusion. While iSCSI
will certainly work over an existing Ethernet network using
hardware, storage and software already on hand, it's vital to keep
iSCSI traffic off the main production network. Not only can iSCSI
traffic congest an everyday network, it's necessary to keep
sensitive corporate data segmented from the general user
population. "It is a SAN, and you don't want data essentially
'leaking out' of the data center," Foskett says.
Right now, the "sweet spot" for iSCSI is the midmarket. Where
iSCSI is most readily embraced is in a Windows storage environment
by organizations that have not previously implemented a SAN --
typically SMBs. However, iSCSI is also making inroads in the
enterprise within the department or large workgroup where it can
serve as primary storage to support Exchange or Oracle databases,
and other block applications. Larger enterprises are even adopting
iSCSI as secondary or even tertiary storage. "It's being used as an
alternative to direct attached, Fibre Channel SAN attached, or even
NAS
[network attached storage]," says Greg
Schulz, founder and senior analyst with the StorageIO Group.
Implementing iSCSI SANs
It takes three components to establish an iSCSI SAN: a network,
a target and an initiator. The network component is relatively
easy; virtually any IP network, such as Ethernet, will work. ISCSI
is ideally suited to
Gigabit Ethernet (GigE) and faster
deployments, but it's crucial to keep the iSCSI SAN separate
from the production LAN through a
VLAN. Either that, or implement a different
LAN segment and switch just for the iSCSI SAN. This separates
storage and user traffic, and reins storage within the SAN.
The "target" is basically a storage destination. In the early
days of iSCSI, routers were popular targets, allowing iSCSI hosts
to access FC storage. Although this approach is still a viable
transition technology, it has largely been abandoned, due to the
availability of dedicated hardware and software products. Hardware
iSCSI targets include dedicated storage arrays from vendors such as
EMC Corp., EqualLogic Inc. and Hitachi Data Systems.
Today it is increasingly common to use a software-based iSCSI
target running on a PC -- turning the server and its storage into
an iSCSI target. Many open source products can be applied to Linux
and BSD environments. Commercial software can also be employed,
including iSCSI Storage Server for Windows from FalconStor Software
Inc., SANmelody from DataCore Software Corp. and Microsoft's iSCSI
target software, released as part of Windows Storage Server.
We should emphasize that there is no such thing as an iSCSI hard
drive; any disk, such as SATA, in a server or array fitted with
target software can be treated as "iSCSI storage." According to
Schulz, "You can roll your own iSCSI storage, or you can redeploy a
server to be an iSCSI array."
Each system that will access iSCSI storage will also need a
hardware or software "initiator." Today, free iSCSI initiator
software is already available for download with Windows 2000,
Windows 2003 and Windows XP Pro. Software is the most popular
initiator choice. "The majority, maybe 90% to 95% of all iSCSI
implementations, involve a software initiator," Schulz says.
Hardware initiators are also available as controller cards from
many FC and
TCP/IP offload engine (TOE) controller
vendors, such as QLogic Corp. and Broadcom Corp.
Areas of concern
Most analysts and users agree that iSCSI is easier to deploy and
manage than FC -- especially in the IP network and initiator. Some
training and configuration may be needed to optimize iSCSI targets,
but this depends on the target. A dedicated iSCSI array may demand
more training and management than an iSCSI server deployed
in-house. Ultimately, an iSCSI storage administrator will still
need to be concerned with LUNs, volumes, provisioning and other
common SAN issues. Pilot deployments can often help organizations
identify potential problem areas and determine the tangible value
of iSCSI.
Network architecture and performance is also critical to iSCSI.
The technology is not tolerant of network interruptions, and an
iSCSI array can easily crash as a result of interruptions or
bottlenecks. Consequently, an iSCSI implementation should include
an aluation of network performance and reliability. Old or outdated
hardware components should be overhauled to improve network
performance or ease bottlenecks, and failover measures should be
deployed wherever possible to ensure network reliability.
Typically, iSCSI is not recommended for use in large transactional
environments where performance is critical.
ISCSI storage performance is also related to the number of
spindles (drives) available in the network. you can achieve
excellent iSCSI storage performance by load balancing more storage
nodes (clustering) rather than placing more drives into fewer large
iSCSI boxes. "Most high-end iSCSI arrays balance I/O across many
more spindles than a Fibre Channel array," Foskett says. "If you
look at the performance tests, you'll see that iSCSI outperforms
Fibre Channel [due to the design of the storage array]."
The future of iSCSI SANs
ISCSI will have to find a way to flourish in the shadow of NAS,
which is currently easier to deploy and manage -- particularly when
coupled with virtualization techniques. But analysts say that iSCSI
has a bright future in the SMB and enterprise, mainly because there
is so much potential for growth. According to Foskett, about 80% of
all servers (primarily low-end Windows servers) are not connected
to shared storage, so iSCSI can expand shared storage dramatically
by interconnecting that untapped wealth of storage.
The eventual move to
10 GigE and faster wireless networking
standards will also impact iSCSI further in the future, enabling
additional bandwidth for iSCSI and ultimately challenging the
dominance of FC. "We're just scratching the tip of the iceberg
with iSCSI," Schulz says. "[The year] 2008 is when I think iSCSI
will really start to come into its prime." ***