CompTIA® Server+ Exam Notes : Features Of Various Storage Technologies

3. Storage

3.3 Features of various storage technologies

Internet Small Computer System Interface (iSCSI): iSCSI is a way of connecting storage devices over a network using TCP/IP. It can be used over a local area network (LAN), a wide area network (WAN), or the Internet. iSCSI devices are disks, tapes, CDs, and other storage devices on another networked computer that you can connect to.

Simply put, iSCSI is an Internet Protocol (IP)-based storage networking standard for linking data storage facilities, developed by the Internet Engineering Task Force (IETF). By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. iSCSI is a data transport protocol used to carry block-level data over traditional Ethernet networks and the Internet. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. Because of the ubiquity of IP networks, iSCSI can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval.

The following are the notable points when compared with FC (Fiber Channel) or FcoE (Fiber Channel over Ethernet):

1. iSCSI is a storage networking protocol built on top of the TCP/IP networking protocol. It does not specifically require any extra hardware, which makes it comparatively inexpensive to implement.

2. FC is a dedicated storage networking architecture that was standardized in 1994. It requires its own networking hardware to implement.

3. FCoE uses standard multipurpose Ethernet networks to connect servers with storage. Unlike iSCSI, it does not run over TCP/IP - it has its own Ethernet protocol occupying a space next to IP in the OSI model.

4. Even though FCoE runs over the same general-purpose switches that iSCSI does, it experiences significantly lower end-to-end latency due to the fact that the TCP/IP header doesn't need to be created and interpreted. The dis-advantage is that it cannot be routed over a TCP/IP WAN.

5. iSCSI eliminates distance limitations imposed by SCSI transfers as it uses TCP/IP.

6. iSCSI is susceptible to network congestion as it uses the same networking channel used by other TCP/IP communications.

7. iSCSI is less expensive compared with Fiber Channel deployments. iSCSI is useful in smaller, quick deployments whereas FC is recommended in larger deployments.

8. Fibre Channel, or FC, is a high-speed network technology (commonly running at 2, 4, 8 and 16-gigabit per second rates) primarily used to connect computer data storage.

9. Fiber channel utilizes FCP (Fibre Channel Protocol ) transport protocol that transports SCSI commands over Fibre Channel networks.

10. Fiber channel provides fastest speeds and SCSI follows Fiber channel in speed.

LUN: LUN stands for Logical Unit Number. LUN is used to identify a logical device that is an independent functional part of a SCSI device. SCSI-2 specification allows 8 logical units for each SCSI device address. The logical unit addresses are numbered 0 to 7. In computer storage, a logical unit number, or LUN, is a number used to identify a logical unit, which is a device addressed by the SCSI protocol or Storage Area Network protocols which encapsulate SCSI, such as Fibre Channel or iSCSI.

Ultra SCSI-3: 8-bit and 16-bit versions, both with 1.5 meter cable length. The 8-bit version supports up to 8 devices. The 16-bit version doubled the number of devices, and supports up to 16 devices (this includes the SCSI board itself).

Ultra-2 SCSI: 8-bit bus; 12 meters; 40 MBps; 8 devices.

Wide Ultra-2 SCSI: 16-bit bus; 12 meters; 80 MBps; 16 devices.

Traditional SCSI has been superseded by later attachment protocols such as Serial Attached SCSI (SAS) and iSCSI, which build on the earlier SCSI command structure.

SCSI Type Transfer speed Bus width
SCSI-1 5MBPS 8 bit bus
Fast Wide SCSI2 20MBPS 16 bit bus
Wide Ultra SCSI3 40MBPS 16 bit bus
Wide Ultra 2 80MBPS 16 bits bus
Narrow Ultra 2 40MBPS 8 bits bus
Ultra160 60MBPS 16 bits bus
Ultra320 320MBPS 16 bits bus
Ultra640 640 MBPS 16 bits bus

SCSI ID - 0=bootable drive, 7=controller, 1-6 = any other devices

SCSI 3 ultra-wide - 16 devices, 0=bootable drive, 15 = controller

Note: The table is given for historical review of SCSI standards, and need not be remembered for the exam.

Host Bus Aapter (HBA): HBA connects a host system (the computer) to other network and storage devices. The terms are primarily used to refer to devices for connecting SCSI, Fibre Channel and eSATA devices, but devices for connecting to IDE, Ethernet, FireWire, USB and other systems may also be called host adapters.

Auto Loader: An auto loader holds ten to twelve tapes, and usually only one tape drive. This is useful when the data to be backed up is more than one tape's capacity. It allows the admin to program and attend to other important works than wait for changing the tape.

Tape Library: A Tape Library contains multiple drives in it. As a result, multiple backups can be taken simultaneously. This is useful when you need to take huge backups in short time. However, remember that if the backups are taken across the network, the network speed limitations will come into play.

SAN(Storage Area Networks): SAN is a high-speed network of storage devices that also connects those storage devices with servers. It provides block-level storage that can be accessed by the applications running on any networked servers. SAN storage devices can include tape libraries, and, more commonly, disk-based devices, like RAID hardware.

The important features of SAN (Storage Area Networks) and NAS (Network Attached Storage) are given below:

1. Only server class devices with SCSI Fibre Channel can connect to the SAN.

2. SAN uses Fibre Channel and based on SCSI protocol.

3. A SAN addresses data by disk block number and transfers raw disk blocks.

4. Backups and mirrors require a block by block copy, even if blocks are empty. A mirror machine must be equal to or greater in capacity compared to the source.

NAS (Network Attached Storage): Networked storage was developed to address the challenges inherent in a server- based infrastructure such as direct-attached storage.

A network-attached storage (NAS) device is a server that is dedicated to nothing more than file sharing. NAS does not provide any of the activities that a server in a server-centric system typically provides, such as email, authentication or file management.

Important features of NAS

1. NAS uses TCP/IP Networks: Ethernet, FDDI, ATM

2. Typically, any machine that can connect to the LAN can use NFS, CIFS or HTTP protocol to connect to a NAS and share files.

3. Backups and mirrors are done on files, not blocks, for a savings in bandwidth and time.


NAS copies files whereas SAN works by copying blocks of data. Even blocks with no data are copied using SAN. Hence NAS requires less bandwidth compared to SAN.

Direct-attached storage (DAS): DAS refers to a digital storage system directly attached to a server or workstation, without a storage network in between. It is a acronym, mainly used to differentiate non-networked storage from the concepts of storage area network (SAN) and network-attached storage (NAS). DAS is ideal for localized file sharing in environments with a single server or a few servers - for example, small businesses or departments and work groups that do not need to share information over long distances or across an enterprise.

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