Introduction to SAN (Storage Area Networks)


A SAN is a high-speed (with the future of 10Gb/s and more) network with heterogeneous (mixed vendor or platforms) servers accessing a common or shared pool of heterogeneous storage devices.

A SAN is a network composed of many servers, connections, and storage devices, including disk, tape, and optical storage. The storage can be located far from the servers that use it.

One server or many heterogeneous servers can share a common storage device, or many different storage devices.
SAN components include:

  • Client access to the LAN and SAN levels.
  • Servers connected to switches or hubs that connect to storage.
  • Storage connected to switches or hubs that connect to servers.
  • Routers or bridges that connect and interface with tape libraries or backup devices.

A SAN is different from traditional networks because it is created from storage interfaces.

  • SAN solutions use a dedicated network behind the servers and are based on primarily Fibre Channel architecture.
  • Fibre Channel provides a highly scalable bandwidth over long distances. Fibre Channel has the ability to provide full redundancy, including switched parallel data paths to deliver high availability and high performance.
  • Clients with business-critical data and applications are concerned about high availability. Fibre Channel SANs help provide the no-single-point-of–failure configurations that business-critical customers require by being able to mirror data or cluster servers over a SAN.

Therefore, a SAN can avoid network bottlenecks. It supports direct, high-speed transfers between servers and storage devices in the following methods:

  • Server to storage— This is the traditional method of interaction with storage devices. The SAN advantage is that the same storage device can be accessed serially or concurrently by multiple servers.
  • Server to server —This provides high-speed, high-volume communications between servers.
  • Storage to storage — In this configuration of a SAN, a disk array could back up its data directly to tape across the SAN, without server processor intervention. A device could be mirrored remotely across the SAN for high-availability configurations.

Storage solution comparison table

Applications Any File serving Storage for application servers
Server and operating systems General purpose Optimized General purpose
Storage devices Internal or external dedicated External direct-attached External shared
Management Labor intensive Centralized Centralized
Data centers Workgroup or departmental Workgroup or departmental Small workgroup to enterprise data centers
Performance Network traffic Increased network performance Higher bandwidth
Distance None Limited distances Greater distances
Speed Bottlenecks Improved bottlenecks Greater speeds
High availability Limited Limited Offers no-single-point-of-failure storage and data path protection
Cost Low cost Affordable Higher cost, but greater benefits

SAN environments provide any-to-any communication between servers and storage resources, including multiple paths.

Benefits of the FC SAN:

Performance — Distance and speed

Efficiency — Reliability and no disruptive scalability

Manageability — More devices supported with less people

Connectivity — Any-to-any connections

Cost effectiveness — Server less backups and tape library sharing

Modular scalability — Dynamic capacity

Consolidated storage — Sharing of centralized storage

SAN components:

A SAN consists of the following hardware and software components:

Switches: A Fibre Channel switch creates the fabric of the SAN. By interconnecting switches, you can create scalable SANs with thousands of port connections.

Routers, bridges, and gateways: Router functionality provides high levels of scalability, dynamic device sharing, and Fibre Channel network fault isolation. Routers, bridges, and gateways extend the SAN over long distances and enable integration of multi-protocol technologies.

Storage devices: A SAN can integrate multiple storage system types, such as disk arrays and tape libraries, to nlallocate storage efficiently.

Servers and HBAs: HBAs connect the server to the SAN. HBA drivers provide an intelligent interface to the switches and minimize CPU overhead.

Cabling and cable connectors: Fiber optic cables provide the physical connections between SAN components.

• SAN management applications:  Applications manage and monitor components and ensure optimal SAN operation.

The parts of a SAN are:

Client layer — The clients are the access point of a SAN.

Server layer — The major components in this layer are the servers, the HBAs, including the GBICs, and the software drivers that enable HBAs to communicate with the fabric layer.

Fabric layer — This is the middle layer of a SAN, the network part of a SAN, where hubs and switches tie all the cables together into a logical and physical network.

Like Hubs or switches, Bridges and multiplexers, Routers, SAN software, Fibre Channel cables.

Storage layer — This is where all the data resides on the disk drives.

SAN infrastructure

You use fabric switches to create the SAN communication paths. The number of storage systems that can be connected is determined by the number of ports available and other hardware constraints. SANs enable expansion by scaling storage capacity across numerous systems and long distances. Scaling increases the number of devices and connections in a SAN. You can increase the number of switches in a fabric, or you can use routing technology to connect multiple SAN fabrics or multiple VSANs.


A fabric is a single switch or a set of switches connected to form a network. Fabric services manage device names and addresses, timestamps, and other functionality for the switches. A set of switches can be connected as a single fabric, an interconnected network of independent fabrics (LSANs for B-series), or partitioned into multiple logical fabrics (Virtual Fabrics for B-series or VSANs for C-series).

SAN scaling

You can increase SAN connectivity by adding switches to an existing SAN or by using switches with more ports. When designing a SAN, you must ensure compliance with Fibre Channel standards and switch specifications. For switch-based scaling, consider the following factors:

Fibre Channel architecture

Fibre Channel supports a maximum of 239 switches in a single fabric. HP specifies support based on rules for the maximum number of switches and maximum number of ports in a single fabric or multi-fabric SAN. Using many switches to obtain a high number of ports is unacceptable if the fabric exceeds the total switch count limit. Likewise, using large-capacity switches can create a network that exceeds the maximum number of ports.

Supported configurations

Each Fibre Channel switch product line specifies the maximum number of ISLs, user ports, and hop counts, as well as link distances and other configuration limitations. The supported configurations determine the practical size of a SAN.

Fabric services

Fabric services are distributed throughout the SAN to coordinate functions among all switches in the fabric. A large SAN requires the management functions provided by high-end switches. Some low-end switches have a limited capacity for expansion. Routing technology facilitates SAN expansion beyond the capacity offered by switch-based scaling.

SAN servers:

After the client layer, the first SAN component that enhances performance is the implementation of the SAN server.

Dataless servers – Data is moved from the servers to storage devices. This enables the servers to perform better because they now manage less data and can handle the server tasks. Its the data that is important to the business.

Clustering – SANs support server clustering. Clusters are a set of independent network servers working together to provide fault tolerance. The services, applications, and resources running on any node in the cluster are available to all connected network users. Clusters are invisible to users and interact as though each were a single server.

Role of the server –

1. Serve as the access point for the clients.

2. Provide load balancing and data caching to improve performance.

3. Schedule backups.


The next component in a SAN is the installation of the HBA in the server.

An HBA provides hard-coded, 64-bit World Wide Name (WWN) and World Wide Port_Name (WWPN) addresses to a SAN device and its ports, and provides more functionality than NICs.

HBAs compared to NICs : HBAs are similar to NICs used in LANs and other non-SAN networks. They replace the traditional SCSI cards and interconnect SAN devices, such as servers and storage devices.

Role of the HBA

The HBA in the SAN provides initialization of Fibre Channel devices and ports that belong to an arbitrated loop or fabric.

HBAs also provide: the Support to the upper-level protocols, such as TCP/IP, ensuring successful interaction between SANs and connected LANs. And encoding of data as per the 8B/10B scheme, which is a fast, extremely secure, and reliable data-encoding mechanism.

Fibre Channel hubs and switches


The hubs on a storage network are used to implement the ring-like Fibre Channel Arbitrated Loop (FC-AL) topology. Unlike the hubs used in traditional networks, a typical Fibre Channel hub can support up to 126 nodes. Hubs have 7 to 12 ports that can be used to connect devices in a Fibre Channel configuration.


Switches provide many more connections than hubs and are used in FC-AL and switched fabric configurations. They offer 8 to 16 ports, and a single switch alone enables the creation of a small-scale SAN. Switches offer a dedicated bandwidth of 100Mb/s and above for each port, enabling frames to be routed between SAN nodes at high speeds.

A switch is identified by its function in a SAN:

• Core (or director)—Provides ISLs for any-to-any connectivity

• Edge (or fabric or SAN)—Provides user ports for connecting servers and storage systems

For some switches, the model name (for example, HP StorageWorks Core Switch 2/64) indicates its intended use in a SAN.

Fibre Channel switches are divided into three categories:

1)     Loop,   2) Fabric,         3) Directors.

Loop switches — These switches are comparatively low cost. These are used to connect an FC-AL loop to the rest of the fabric.

Fabric switches — These switches are expensive and are predominantly used to implement the switched fabric topology.

Directors — This is the most expensive category of switches, but they offer the best performance and maximum reliability. The estimated annual downtime for a director is barely five minutes.

Other advanced services provided by Fibre Channel switches are:

Buffer-to-buffer flow control during transactions Services, such as:

Fabric Login (FLOGI) — Enables nodes to be successfully initialized (allocated a unique address) in a switched environment, enabling communication between two nodes

Simple Name Server (SNS) — Helps a source node to discover the destination node within the fabric without causing unnecessary communication overhead

Registered State Change Notification (RSCN) that notifies Fibre Channel nodes about the changes in the existing topology

Gigabit technology.

Can address more devices than SCSI or NIC counterparts.  Provide I/O connectivity to more devices over longer distances than SCSI.  Provide the ability for Fibre Channel frames to relay over gateways.

Storage devices

Storage can be based on SCSI or Fibre Channel.

Fibre Channel-based storage enables direct connection to the Fibre Channel network, providing distance and speed enhancements over SCSI.

Storage arrays

The storage array is an external drive enclosure containing a controller, power supply, fan assembly, and disk drive housings. A single array holds several drives, producing an enormous amount of storage in a compact unit. Some array components can be replaced, so that installation and replacement does not require downtime. The series of fixed disk drives are logically addressed as a single, larger drive.

Backup devices

Backup and recovery system configurations can range from an external tape drive attached to the corporate server, to large tape libraries capable of handling hundreds of backup media.

Typically there are two types of back up devices named Tape Library and Autoloader.

Tape libraries:

A tape library is a high-capacity data storage system for storing, retrieving, reading, and writing multiple magnetic tape cartridges. They meet the performance and capacity characteristics of a SAN environment.


Autoloaders are ideal for applications that require high-capacity, high-speed tape backup where physical space, backup time, and personnel resources are limited.

Management of the SAN:

SAN software is required to manage and troubleshoot a SAN environment.

SAN software is used to manage: Hardware and software within the SAN (such as storage, switches, and hubs), Multiple storage enclosures, Multiple operating systems, Multiple vendors.

Resources to be managed in different geographical locations

SAN management functions include: Device installation, configuration, and monitoring, SAN resource inventory, Report utilities, Automated component and fabric discovery, Management of fabric configuration, Name services, Security management, Performance monitoring and load balancing.

At the storage level SAN management includes the management of: Disks and disk arrays, Tapes and tape libraries, Cabling, Hubs, switches, gateways, bridges, and routers, Interswitch links, HBAs.

SAN software uses the SCSI enclosure services that might impact FC-AL bandwidth, Simple Network Management Protocol (SNMP)

And at the enterprise level SAN software manages: Storage, Servers,.

Fibre Channel technology

Fibre Channel is a comprehensive set of standards for communication among servers, storage systems, and peripheral devices. A Fibre Channel network provides connectivity among heterogeneous devices and supports multiple interconnect topologies.

The network can be connected to a variety of storage systems:

• RAID arrays

• Tape devices and backup libraries

Fibre Channel technology supports simultaneous use of these transport protocols:

• IP



Hosts and applications see storage devices attached to the SAN as if they are locally attached storage. Multiple protocols and a broad range of devices can be supported. Connections can be either optical fiber (for distance) or copper cable links (for short distance at low cost).


Fibre Channel uses three protocols:

Point-to-point — Devices are directly connected to other devices without the use of hubs, switches, or routers.

Fibre Channel Arbitrated Loop (FC-AL) — FC-AL has a shared bandwidth, distributed topology, connects with hubs, and is the simplest form of a fabric topology.

Fibre Channel Switched Fabric (FC-SW) — FC-SW provides the highest performance and connectivity of the three topologies. It has nondisruptive scalability and switch connection.

Fibre Channel supports 126 nodes on an FC-AL, and 16 million nodes on an FC-SW and provides connectivity over several kilometers (up to 10km) when using optical fiber.

Fibre Channel has features that include:

Performance, Hot swap/plug, Reliable, Multiple topology,

Longer cable lengths , Gigabit bandwidth and backward compatible.

Loop resiliency, Scalability, Congestion-free.

Set of storage network services like discovery, addressing, zoning, failover, management, and security.

Fibre Channel ports and description

N_port – All node (server or storage) ports are called N_Ports. An N_Port attaches to an F_Port in a point-to-point protocol. N-port to N-port is uncommon, so when two nodes are direct-attached it is through an arbitrated loop (NL_Port to NL_Port).

L_port – All loop-hub ports are called L_Ports, which stands for loop ports.

NL_port – An N_Port that contains arbitrated loop functions associated with arbitrated loop topology is called an NL_Port.

F_port – The F_Port, or fabric port, is the Link_Control_Facility within the fabric (switch) that attaches to an N_Port.

FL_port – An F_Port, that contains arbitrated loop functions associated with arbitrated loop topology is called an FL_Port, which stands for fabric loop port.

E_port – An E_Port is used for connecting fabrics (switches). The link is called the inter-switch link (ISL).

G_port – A G_Port (generic port) can auto-discover its type. It automatically configures itself as an E, N, or NL port.

Fibre Channel addressing is identified by:

AL_PA — A 24-bit address that identifies a port.

Port address — A subset of the WWN used by Fibre Channel networks that assign N_port and NL_port addresses.

WWN — A unique 64-bit address assigned to each node. If a node has more than one port, each port is assigned a WWN.

SNS — A server located on the switch that is used to keep track of WWNs and the associated 24-bit N_Port IDs.

FLOGI — A protocol used between a node (server or storage) and the fabric to establish identification and capability parameters for subsequent communication.


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