Fibre Channel Disk Attachment - IBM DS3000 Introduction And Implementation Manual

7065DiskAttach.fm

1.1 Fibre Channel disk attachment

Fibre Channel (FC) is a high-speed disk attachment technology, designed to connect a large
number of storage devices to a number of host servers across a Storage Area Network
(SAN). Fibre Channel Protocol (FCP) transfers SCSI commands and data across physical FC
links.
FC supports a much higher number of devices and much longer cable lengths than SCSI. It
has become the preferred disk attachment technology in midrange and large scale datacenter
solutions.
At the time of writing, the maximum FC throughput is 4Gbps. In fact, 10 Gbps links can be
used today, but only for SAN switch interconnection.
Host servers contain one or more FC Host Bus Adapters (HBA). The HBAs provide
connectivity to the storage devices using FC cabling.
For more information on Fibre Channel and SANs, see Introduction to Storage Area
Networks, SG24-5470.
FC topologies
FC supports several connectivity topologies:
Point-to-point
This is the simplest topology and provides a direct link between a FC HBA inside a host
server and a storage device.
Arbitrated loop
This topology can be used to interconnect several FC devices. A typical example would be
to attach a certain number of host servers to a FC storage subsystem. A loop can consist
of up to 126 devices.
Devices on the loop use one-way ring communication. In any given moment, only two
devices on the loop can communicate. This means the devices share bandwidth, so the
arbitrated loop topology is not suitable for high performance requirements.
Arbitrated loops were commonly implemented with the use of a FC hub. Even though this
is physically a star topology, logically it will be a loop. Alternatively, devices can be
connected in a daisy chain manner.
Arbitrated loops are rarely seen nowadays, as switched fabrics have become the norm.
Switched fabric
The most commonly used topology in a typical SAN today is switched fabric. SAN
switches are used to provide FC connectivity between the host servers and storage
devices. Switched fabrics can become very complex in large scenarios, connecting
hundreds of host servers to a very large number of storage subsystems.
SAN switches provide optimized traffic flow and increased performance by allowing
concurrent data transfers between many connected hosts and storage devices. Switched
fabrics can provide dedicated bandwidth, as opposed to arbitrated loop technology, where
the bandwidth is shared among all the devices in the loop.
FC protocol layers
The FC protocol is split into five layers, named FC0 through FC4. Let's take a brief look at
these layers:
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IBM System Storage DS3000: Introduction and Implementation Guide
Draft Document for Review August 30, 2007 12:59 am