Tercera fase del proyecto: Evaluación categorial y dimensional de la

Fibre Channel is an open, technical standard for networking. It incorporates the data delivery (OSI Transport layer) characteristics of an I/O bus with the flexible connectivity and distance characteristics of a network. One of the fundamental differences of SAN attached storage, compared to NAS, is that SAN storage systems typically attach directly to the network by means of hardware called host bus adapters (HBA). NAS, on the other hand, requires a “front-end” server as part of the appliance, which attaches to the LAN by means of a Network Interface Card (NIC).

A SAN eliminates the traditional dedicated connection between a server and DAS. Individual servers no longer “own and manage” the storage devices. Restrictions to the amount of data that a server can access is also minimized. Instead, a SAN enables many heterogeneous servers to share a common storage “utility”. This utility may comprise many storage devices, including disk, tape, and optical storage, and may be located many kilometres from the servers which use it. Thus SAN attached storage has the potential to be highly scalable relative to a typical NAS device.

Because of its channel, or bus-like, qualities, hosts and applications see storage devices attached to the SAN as if they are locally attached storage. With its network characteristics, it can support multiple protocols and a broad range of devices, and it can be managed as a network.

Fibre Channel is a multi-layered network, based on a series of American National Standards Institute (ANSI) standards. These define characteristics and functions for moving data across the network. Like other networks, information is sent in structured packets or frames, and data is serialized before transmission. But, unlike other networks, the Fibre Channel architecture includes a significant amount of hardware processing. This is oriented to storage block I/O protocols, such as serial SCSI (known as FCP). A SAN is therefore capable of delivering very high performance relative to a NAS device, which is optimized for network file I/O. The speed currently achieved is 100 MBps full duplex, with 200 MBps soon to be delivered.

Measured effective data rates of Fibre Channel have been demonstrated in the range of 60 to 80 MBps over the 1 Gbps implementation. This compares to less than 30 MBps measured over Gigabit Ethernet. The packet size of Fibre Channel is 2,112 bytes (rather larger than some other network protocols). For instance, an IP packet is 1,518 bytes, although normally IP transfers are much smaller. But for Fibre Channel a maximum transfer unit sequence of up to 64 frames can be defined, allowing transfers of up to 128 MB without incurring additional overhead due to processor interrupts. Thus, today Fiber Channel is unsurpassed for efficiency and high performance in moving large amounts of data.

Transmission is defined in the Fibre Channel standards across three transport topologies:

򐂰 Point-to-point: This is a bi-directional, dedicated interconnection between two nodes. This delivers a topology similar to DAS, but with the added benefits of longer distance.

򐂰 Arbitrated loop: This is a uni-directional ring topology, similar to a token ring, supporting up to 126 interconnected nodes. Each node passes data to the next node in the loop, until the data reaches the target node. All nodes share the 100 MBps bandwidth. Devices must arbitrate for access to the loop. FC-AL is suitable for small SAN configurations, or SANlets.

򐂰 Switched fabric: This describes an intelligent switching infrastructure which delivers data from any source to any destination. Each node is able to utilize the full 100 MBps bandwidth. Each logical connection receives dedicated bandwidth, so the overall bandwidth is multiplied by the number of connections. Complex fabrics must be managed by software which can exploit SAN management functions which are built into the fabric. A mix of these three topologies can be implemented to meet specific needs.

Figure 1-10 SAN: The network behind the servers

SAN supports the following direct, high speed transfers:

򐂰 Server-to-storage:This is similar to a DAS connection to a server. The SAN advantage, as with a NAS appliance, is that the same storage device may be accessed serially or concurrently by multiple servers.

򐂰 Server-to-server:This is high-speed communications between servers. 򐂰 Storage-to-storage:Outboard data movement means data can be moved

with limited server intervention. Examples include a disk device moving data directly to a tape device, or remote device mirroring across the SAN.

Fibre Channel combines the characteristic strengths of traditional I/O channels with those of computer networks, in the following specifics:

򐂰 High performance for large data transfers by using storage transport protocols and extensive hardware assists

򐂰 Serial data transmission

򐂰 A physical interface with a low error rate definition

򐂰 Reliable transmission of data with the ability to guarantee or confirm error free delivery of the data

򐂰 Packaging data in packets (frames in Fibre Channel terminology)

Local Area Network

Server-to-server Server-to-storage Storage-to-storage Storage Area Network

򐂰 Flexibility in terms of the types of information which can be transported in frames (such as data, video, and audio)

򐂰 Use of existing device-oriented command sets, such as SCSI

򐂰 A vast expansion in the number of devices which can be addressed when compared to traditional I/O interfaces

It is this high degree of flexibility, availability, and scalability, over long distances, and the broad acceptance of the Fibre Channel standards by vendors throughout the IT industry, which make the Fibre Channel architecture attractive as the basis for new enterprise storage infrastructures.