Fibre Channel for SANs.pdf

Source: Fibre Channel for SANs

Chapter

Fibre Channel and

Storage Area Networks

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Fibre Channel and Storage Area Networks

Chapter 1

Introduction

Fibre Channel technology is over a decade old. How successful has it been?

Here is an illustration. The first edition of this book included a section

called “The Unification of LAN and Channel technologies,” which

described how Fibre Channel would be part of a trend towards convergence

between LANs and channels. LANs (Local Area Networks) are used for

computer-to-computer communications, and channels are high-efficiency,

high-performance links between computers and their long-term storage

devices (disk and tape drives), and other I/O devices.

Since then, the prediction has come true, in three quite different ways.

• Most important has been the introduction and widespread use of the term

“Storage Area Network,” or SAN, describing a network which is highly

optimized for transporting traffic between servers and storage devices.

• At the physical layer, the LAN and Fibre Channel technologies have

become nearly identical — Gigabit Ethernet and Fibre Channel share com-

mon signaling and data encoding mechanisms, and the future 10 Gb/s

Ethernet and Fibre Channel are expected to share nearly the same data rate.

• The management methods for Fibre Channel SANs have steadily

approached the traditional methods used for LAN management, although

the current level of management effort required for Fibre Channel SANs is

still higher than for LANs.

Interestingly, however, although the LAN and SAN types of computer

data communications have converged at a technology level, they have so far

stayed quite different in how they are used and how they are managed. That

is, systems are usually built with the SAN storage traffic separated on sepa-

rate networks from the LAN traffic, so that the management, topologies, and

provisioning of each network can be optimized for the types of traffic tra-

versing them.

The trends that originally motivated the creation of Fibre Channel have

continued or accelerated. The speed of processors, the capacities of memory,

disks, and tapes, and the use of switched communications networks have all

been doubling every 18 to 24 months, and the doubling period has in many

cases even been steadily shortening slightly. However, the rate of I/O

improvement has been much slower, so that devices are even more I/O lim-

ited. The continuing observation is that computers usually appear nearly

instantaneous, except when doing I/O (e.g., downloading web pages), or

managing stored data (e.g., backing up file systems).

Fibre Channel, and Storage Area Networks, are focused at (a) optimizing

the movement of data between server and storage systems, and (b) managing

the data and the access to the data, so that communications are optimized as

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Fibre Channel and Storage Area Networks

much as possible, while continuously and reliably providing access to data,

for whoever needs it.

Fibre Channel Features

Following is a list of the major features that Fibre Channel provides:

• Unification of networking and I/O channel data communications: This was

described in detail above, and allows storage to be decoupled from servers

and managed separately. Similarly, many servers can directly access the

data as if it were their own, as long as they are coordinated to manage it

coherently.

• Bandwidth: The base definition of Fibre Channel provides better than 100

MBps for I/O and communications on current architectures, with speeds

defined up to 4 times this rate, for implementation as market and applica-

tions dictate.

• Inexpensive implementation: Fibre Channel uses an 8B/10B encoding for

all data transmission, which, by limiting low-frequency components,

allows design of AC-coupled gigabit receivers using inexpensive CMOS

VLSI technology

• Low overhead: The very low 10 -12 bit error rate achievable using a combi-

nation of reliable hardware and 8B/10B encoding allows very low extra

overhead in the protocol, providing efficient usage of the transmission

bandwidth and saving effort in implementation of low-level error recovery

mechanisms.

• Low-level control: Local operations depend very little on global informa-

tion. This means, for example, that the actions that one Port takes are only

minimally affected by actions taking place on other Ports, and that individ-

ual computers need to maintain very little information about the rest of the

network. This feature minimizes the amount of work to do at the higher

levels.

• For example, hardware-controlled flow control alleviates the host pro-

cessors from the burden of managing much of the flow control overhead.

• Similarly, the low-level hardware does sophisticated error detection and

deletion, so that it can assure delivery of data intact or not at all. Upper

layer protocols don’t have to do as much error detection, and can be

more efficient.

• Flexible topology: Physical connection topologies are defined for (1)

point-to-point links, (2) shared-media loop topologies, and (3) packet-

switching network topologies. Any of these can be built using the same

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Fibre Channel and Storage Area Networks

Chapter 1

hardware, allowing users to match physical topology to the required con-

nectivity characteristics.

• Distance: 50 m in a room simplifies wiring, more important is 10 km,

which allows remote copy without WAN infrastructure. Consider a high

performance disk drive attached to a computer over an optical fiber. The

access time for the disk drive (to rotate the disk and move the head over the

data) would be roughly 5 ms. The speed of light in optical fiber is about

124 mi/ms. This means that the time to reach an optically connected disk

drive located a mile away would be only 0.008 ms more than the time to

reach a disk drive in the same enclosure.

• Availability: More capability to attach to multiple servers allows the data

to be accessed through many paths, which enhances availability in case

one of those paths fails.

• Flexible transmission service: Mechanisms are defined for multiple

Classes of services, including (1) dedicated bandwidth between Port pairs

at the full hardware capacity, (2) multiplexed transmission with multiple

other source or destination Ports, with acknowledgment of reception, and

(3) best-effort multiplexed datagram transmission without acknowledg-

ment, for more efficient transmission in environments where error recov-

ery is handled at a higher level, (4) dedicated connections with

configurable quality of service guarantees on transmission bandwidth and

latency, and (5) reliable multicast, with a dedicated connection at the full

hardware capacity.

• Standard protocol mappings: Fibre Channel can operate as a data transport

mechanism for multiple Upper Level Protocols, with mappings defined for

IP, SCSI-3, IPI-3 Disk, IPI-3 Tape, HIPPI, the Single Byte Channel Com-

mand set for ESCON, the AAL5 mapping of ATM for computer data, and

VIA or Virtual Interface Architecture. The most commonly used of these

currently are the mapping to SCSI-3, which is termed “FCP,” and the map-

ping to ESCON, which is termed either “FICON,” or “SBCON,” depend-

ing on context.

• Wide industry support: Most major computer, disk drive, and adapter man-

ufacturers are currently developing hardware and/or software components

based on the Fibre Channel ANSI standard.

These improvements to traditional channels don’t actually provide much

real benefit when a single server is used to process the data on a single stor-

age device. However, when multiple servers act together (for better reliabil-

ity, or higher throughput, or better pipelining, etc.) to work with the data on

multiple storage devices of different types, then the advantages of Fibre

Channel can become very important.

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Fibre Channel and Storage Area Networks

Storage Area Networks

What is a Storage Area Network, and how is it different from the various

other types of networks that are built?

Here is a definition of a Storage Area Network, from one of the leaders in

the industry:

A Storage Area Network (SAN) is a dedicated, centrally managed, secure

information infrastructure, which enables any-to-any interconnection of

servers and storage systems.

This definition is unfortunately not particularly instructive as to, for

example, the difference between SANs and LANs, or MANs, or even

WANs, all of which, in some applications, could fit this description.

The difference between SANs and other types of networks can perhaps

best be understood by considering the difference between the storage and

networking ports on a desktop computer. Every computer has access to some

kind of long-term storage, and almost every computer has access to some

way of communicating with other computers. The storage interface is highly

optimized, tightly controlled (in laptops and most desktop machines, it may

not even be visible outside the box), and not shared with any other comput-

ers — which helps make it highly predictable, efficient, and fast. Network

interfaces, on the other hand, are much slower, less efficient (you have to

wait for them), and have higher overhead, but they allow access to any other

machine that it knows how to communicate with.

Storage Area Networks are built to incorporate the best of both storage

and networking interfaces: fast, efficient communications, optimized for

efficient movement of large amounts of data, but with access to a wide range

of other servers and storage devices on the network.

The primary difference then between a Storage Area Network and the

other types of networks mentioned is that, in a SAN, communication within

the network is well-managed, very well-controlled, and predictable. There-

fore, each entity on the network can almost operate is if it has sole access to

whichever partner on the network that it is currently communicating with.

A primary reason for this has been the idea of decoupling the servers

from their storage, and allowing multiple servers to access the same data at

the same time. The key here is that client systems often access their through

servers, which assure consistency, security, and authorization for the data

access. Clients, however, don’t particularly care which server is used to

access the data, and the data is the same no matter which server is accessing

it. This three-tiered system of clients displaying the data, servers processing

and managing the data, and storage subsystems holding the data, is tied

together with networks — LANs and SANs — between each layer.

Fibre Channel overlaps very little with Ethernet, except in very specific

applications. For general-purpose communications, Ethernet is very difficult

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