Storage

This section briefly describes the main types of storage devices that can be found in the market.

Disk systems

In brief a disk system is a device in which a number of physical storage disks sit side-by-side. By being contained within a single “box”, a disk system usually has a central control unit that manages all the I/O, simplifying the integration of the system with other devices, such as other disk systems or servers.

Depending on the “intelligence” with which this central control unit is able to manage the individual disks, a disk system can be a JBOD or a RAID. Just A Bunch Of Disks (JBOD)

In this case, the disk system appears as a set of individual storage devices to the device they are attached to. The central control unit provides only basic functionality for writing and reading data from the disks.

Redundant Array of Independent Disks (RAID)

In this case, the central control unit provides additional functionality that makes it possible to utilize the individual disks in such a way to achieve higher fault-tolerance and/or performance. The disks themselves appear as a single storage unit to the devices to which they are connected.

Depending on the specific functionality offered by a particular disk system, it is possible to make it behave as a RAID and/or a JBOD; the decision as to which type of disk system is more suitable for a SAN implementation strongly depends on the performance and availability requirements for this particular SAN.

Tape systems

Tape systems, in much the same way as disk systems do, are devices that comprise all the necessary apparatus to manage the use of tapes for storage purposes. In this case, however, the serial nature of a tape makes it impossible for them to be treated in parallel, as RAID devices are leading to a somewhat simpler architecture to manage and use.

There are basically three types of systems: drives, autoloaders and libraries, that are described as follows.

Tape drives

As with disk drives, tape drives are the means by which tapes can be connected to other devices; they provide the physical and logical structure for reading from, and writing to tapes.

Tape autoloaders

Tape autoloaders are autonomous tape drives capable of managing tapes and performing automatic back-up operations. They are usually connected to high-throughput devices that require constant data back-up.

Tape libraries

Tape libraries are devices capable of managing multiple tapes simultaneously and, as such, can be viewed as a set of independent tape drives or autoloaders. They are usually deployed in systems that require massive storage capacity, or that need some kind of data separation that would result in multiple single-tape systems. As a tape is not a random-access media, tape libraries cannot provide parallel access to multiple tapes as a way to improve performance, but they can provide redundancy as a way to improve data availability and fault-tolerance. Once more, the circumstances under which each of these systems, or even a disk system, should be used, strongly depend on the specific requirements that a

Chapter 2. How, and why, can we use a SAN? 17 particular SAN implementation has. However, we can say that disk systems are usually used for online storage due to their superior performance, whereas tape systems are ideal for offline, high-throughput storage, due to the lower cost of storage per byte.

In the next section we describe the prevalent connectivity interfaces, protocols and services for building a SAN.

2.3.2 SAN connectivity

SAN connectivity comprises all sorts of hardware and software components that make possible the interconnection of storage devices and servers. In this section, we have divided these components into three sections according to the level abstraction to which they belong: lower level layers, middle level layers, and higher level layers.

Lower level layers

This section comprises the physical data-link, and the network layers of connectivity.

Ethernet interface

Ethernet adapters are typically used on conventional server-to-server or workstation-to-server network connections. They build up a common-bus topology by which every attached device can communicate with each other, using this common-bus for such. An Ethernet adapter can reach up to 10 Gbps of data transferred.

Fibre Channel

Fibre Channel (FC) is a serial interface (usually implemented with fiber-optic cable, and is the primary architecture for the vast majority of SANs. To support this there are many vendors in the marketplace producing Fibre Channel adapters, and other FC devices. One of the reasons that FC is so popular is that it allows the maximum SCSI cable length of 25 meters restriction to be overcome. Coupled with the increased speed that it supports, it quickly became the

connection of choice.

Note: With respect to data throughput speeds, in this redbook we use the following representations: 򐂰 1 Gbps = 100 MBps 򐂰 2 Gbps = 200 MBps 򐂰 4 Gbps = 400 MBps 򐂰 8 Gbps = 800 MBps 򐂰 10 Gbps = 1000 MBps

SCSI

The Small Computer System Interface (SCSI) is a parallel interface. SCSI devices are connected to form a terminated bus (the bus is terminated using a terminator). The maximum cable length is 25 meters, and a maximum of 16 devices can be connected to a single SCSI bus. The SCSI interface has many configuration options for error handling and supports both disconnect and reconnect to devices and multiple initiator requests. Usually, a host computer is an initiator. Multiple initiator support allows multiple hosts to attach to the same devices and is used in support of clustered configurations. The Ultra3 SCSI adapter today can have a data transfer up to 160 MBps.

Middle level layers

This section comprises the transport protocol and session layers.

FCP

The Fibre Channel Protocol (FCP) is the interface protocol of SCSI on Fibre Channel. It is a gigabit speed network technology primarily used for Storage Networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards (INCITS), an American National Standard Institute (ANSI) accredited standards committee. It started for use primarily in the supercomputer field, but has become the standard connection type for storage area networks in enterprise storage. Despite its name, Fibre Channel signaling can run on both twisted-pair copper wire and fiber optic cables.

iSCSI

Internet SCSI (iSCSI) is a transport protocol that carries SCSI commands from an initiator to a target. It is a data storage networking protocol that transports standard Small Computer System Interface (SCSI) requests over the standard Transmission Control Protocol/Internet Protocol (TCP/IP) networking technology. iSCSI enables the implementation of IP-based storage area networks (SANs), enabling customers to use the same networking technologies — for both storage and data networks. As it uses TCP/IP, iSCSI is also well suited to run over almost any physical network. By eliminating the need for a second network technology just for storage, iSCSI has the potential to lower the costs of deploying networked storage.

FCIP

Fibre Channel over IP (FCIP) is also known as Fibre Channel tunneling or storage tunneling. It is a method to allow the transmission of Fibre Channel information to be tunnelled through the IP network. Because most organizations already have an existing IP infrastructure, the attraction of being able to link geographically dispersed SANs, at a relatively low cost, is enormous.

Chapter 2. How, and why, can we use a SAN? 19 FCIP encapsulates Fibre Channel block data and subsequently transports it over a TCP socket. TCP/IP services are utilized to establish connectivity between remote SANs. Any congestion control and management, as well as data error and data loss recovery, is handled by TCP/IP services, and does not affect FC fabric services.

The major point with FCIP is that is does not replace FC with IP, it simply allows deployments of FC fabrics using IP tunnelling. The assumption that this might lead to is that the “industry” has decided that FC-based SANs are more than appropriate, and that the only need for the IP connection is to facilitate any distance requirement that is beyond the current scope of an FCP SAN.

iFCP

Internet Fibre Channel Protocol (iFCP) is a mechanism for transmitting data to and from Fibre Channel storage devices in a SAN, or on the Internet using TCP/IP.

iFCP gives the ability to incorporate already existing SCSI and Fibre Channel networks into the Internet. iFCP is able to be used in tandem with existing Fibre Channel protocols, such as FCIP, or it can replace them. Whereas FCIP is a tunneled solution, iFCP is an FCP routed solution.

The appeal of iFCP is that for customers that have a wide range of FC devices, and who want to be able to connect these using the IP network, iFCP gives the ability to permit this. iFCP can interconnect FC SANs with IP networks, and also allows customers to use the TCP/IP network in place of the SAN.

iFCP is a gateway-to-gateway protocol, and does not simply encapsulate FC block data. Gateway devices are used as the medium between the FC initiators and targets. As these gateways can either replace or be used in tandem with existing FC fabrics, iFCP could be used to help migration from a Fibre Channel SAN to an IP SAN, or allow a combination of both.

FICON

FICON architecture is an enhancement of, rather than a replacement for, the now relatively old ESCON® architecture. As a SAN is Fibre Channel based, FICON is a prerequisite for z/OS systems to fully participate in a heterogeneous SAN, where the SAN switch devices allow the mixture of open systems and mainframe traffic.

FICON is a protocol that uses Fibre Channel as its physical medium. FICON channels are capable of data rates up to 200 MBps full duplex, they extend the channel distance (up to 100 km), increase the number of control unit images per link, increase the number of device addresses per control unit link, and retain the topology and switch management characteristics of ESCON.

Higher level layers

This section comprises of the presentation and application layers.

Server-attached storage

The earliest approach was to tightly couple the storage device with the server. This server-attached storage approach keeps performance overhead to a minimum. Storage is attached directly to the server bus using an adapter card, and the storage device is dedicated to a single server. The server itself controls the I/O to the device, issues the low-level device commands, and monitors device responses.

Initially, disk and tape storage devices had no on-board intelligence. They just executed the server’s I/O requests. Subsequent evolution led to the introduction of control units. Control units are storage off-load servers that contain a limited level of intelligence, and are able to perform functions, such as I/O request caching for performance improvements, or dual copy of data (RAID 1) for availability. Many advanced storage functions have been developed and implemented inside the control unit.

Network Attached Storage

Network Attached Storage (NAS) is basically a LAN-attached file server that serves files using a network protocol such as Network File System (NFS). NAS is a term used to refer to storage elements that connect to a network and provide file access services to computer systems. A NAS storage element consists of an engine that implements the file services (using access protocols such as NFS or CIFS), and one or more devices, on which data is stored. NAS elements may be attached to any type of network. From a SAN perspective, a SAN-attached NAS engine is treated just like any other server, but a NAS does not provide any of the activities that a server in a server-centric system typically provides, such as e-mail, authentication, or file management.

NAS allows more hard disk storage space to be added to a network that already utilizes servers without shutting them down for maintenance and upgrades. With a NAS device, storage is not an integral part of the server. Instead, in this storage-centric design, the server still handles all of the processing of data, but a NAS device delivers the data to the user. A NAS device does not need to be located within the server but can exist anywhere in the LAN and can be made up of multiple networked NAS devices. These units communicate to a host using Ethernet and file-based protocols. This is in contrast to the disk units discussed earlier, which use Fibre Channel protocol and block-based protocols to

communicate.

NAS storage provides acceptable performance and security, and it is often less expensive for servers to implement (for example, ethernet adapters are less expensive than Fibre Channel adapters).

Chapter 2. How, and why, can we use a SAN? 21 In an effort to bridge the two worlds and to open up new configuration options for customers, some vendors, including IBM, sell NAS units that act as a gateway between IP-based users and SAN-attached storage. This allows for the connection of the storage device of choice (an ESS, for example) and share it between your high-performance database servers (attached directly through Fibre Channel) and your end users (attached through IP) who do not have performance requirements nearly as strict.

NAS is an ideal solution for serving files stored on the SAN to end users in cases where it would be impractical and expensive to equip end users with Fibre Channel adapters. NAS allows those users to access your storage through the IP-based network that they already have.