A storage area network
(
SAN) organises a broad assortment of storage
devices into a single storage resource that can then be
provisioned, allocated and managed for the
entire enterprise. Although issues like storage capacity,
performance and management often receive the most attention, the
connectivity between each SAN device plays a critical role in
successful SAN deployment. Each
switch and storage system on the SAN must be
interconnected -- usually through optical fiber or copper
cabling -- and the physical interconnections must support
bandwidth levels that can adequately handle
the peak data activities that occur. This overview details the
role of
Fibre Channel (FC),
Ethernet and
iSCSI connectivity on a SAN.
Fibre Channel
FC is the quintessential SAN interconnect and virtually every
storage switch and storage platform provides FC ports. Multiple FC
ports support simultaneous data streams, but individual ports can
often be aggregated into groups for even higher effective
bandwidth. As an example, the
All-In-One Buying Guide notes that the
InServ E200 Storage Server from 3PAR Data Inc. supports up to 12
FC ports, while the TagmaStore AMS1000 from Hitachi Data Systems
Inc. (HDS) provides up to eight FC ports. Servers and other
devices can also be fitted with FC host channel adapters (HCAs)
to enable an FC interface.
As a serial interface, FC bandwidth is denoted in Gbps. Early FC
implementations ran at 1 Gbps per port, and 2 Gbps reigned until
recently. Today, 4 Gbps FC is readily available and 10 Gbps
implementations are appearing on some high-end systems and
director-class switches. FC operates with numerous protocols,
though
SCSI and IP are the most popular
implementations.
FC can use several types of physical media.
Twisted pair cable is used to cover
relatively short distances at low speeds between FC devices.
Coaxial cables generally offer better
shielding against signal interference and can run across
somewhat longer distances.
Optical fiber is routinely used to carry the
fastest signals across distances up to 10 km.
Ethernet
While Ethernet connectivity is generally used on the greater
local area network (LAN), its use in the SAN has been limited by
its relatively slow bandwidth. Traditional Ethernet ports support
10/100 Mbps -- far slower than FC. This had limited Ethernet in the
SAN to basic management tasks. For example, a storage device or
switch might include a single Ethernet port that connects the
device to the LAN where an administrator can manage the device
across it. Ethernet typically uses two protocols;
Transmission Control Protocol (TCP), which
handles the organisation of data into packets, and
Internet Protocol (IP), which handles the
way those data packets are addressed. In fact, the terms
"Ethernet" and "TCP/IP" are often used interchangeably.
Ethernet bandwidth is increasing today, which boosts performance
on the LAN and also makes Ethernet use more practical for carrying
data on the SAN. One
Gigabit Ethernet (GigE) is now common on
many servers and switches, and the eventual emergence of 10 GigE
promises to put Ethernet on par with 10 Gigabit (Gbit) Fibre
Channel.
Traditional Ethernet LAN deployments used coaxial cables, but
twisted-pair cabling (e.g., Category 5 or Category 6 Ethernet
cables) is the most common LAN cabling. Ten GigE often relies on
optical fiber with transmission distances up to 40 km, which makes
the technology far more expensive and limits its use to network
backbones. As copper cabling becomes available for 10 GigE, the
technology should see far more use within data centers and
SANs.
ISCSI and FCIP
Fibre Channel SANs have long been challenged by deployment
expense and management complexity -- usually keeping SANs out of
reach of smaller IT organisations. The recent development of iSCSI
promises to ease these challenges by
encapsulating SCSI commands into IP packets
for transmission over an Ethernet connection, rather than a FC
connection. This approach eliminates FC in favor of Ethernet,
which allows iSCSI to transfer data over LANs, WANs or the
Internet and supports storage management over long
distances.
In actual practice, a user or application will cause the
operating system to generate corresponding SCSI storage commands.
Those SCSI commands and data are then encapsulated and IP headers
are added to make packets. The packets can then be sent over an
ordinary Ethernet connection. The remote end of the iSCSI
connection disassembles the encapsulated content and passes the
SCSI commands to the SCSI controller and storage device. This also
works in reverse, so any data or responses can be sent back to the
user or application across the Ethernet connection.
Another alternative is FCIP. FCIP translates FC commands and
data into IP packets, which can be exchanged between distant FC
SANs. It's important to note that FCIP only works to connect FC
SANs, but iSCSI can run on any Ethernet network. ***