Components of a SAN Storage Solutions | Storage Virtualization | Storage Systems

Storage System Environment

Hi Friends,

Now we will learn about the Storage System Environment, components of a storage system  and Disk drive components. To know more about other important components of Intelligent Storage System.

Basic Introduction

This article provides you an understanding about the various hardware components of a disk drive, disk geometry and disk performance. The connectivity between the host and storage assisted by bus technology and interface protocols is also explained.

Components of a Storage System Environment

The three main components in a storage system environment — the host, connectivity, and storage.

Host - Users store and retrieve data through applications. The computers on which these applications run are referred to as hosts. Hosts can range from simple laptops to complex clusters of servers.

Physical components

A host has three key physical components:
·         Central processing unit (CPU)
·         Storage, such as internal memory and disk devices
·         Input/output (I/O) devices

The physical components communicate with one another by using a communication pathway called a bus. A bus connects the CPU to other components, such as storage and I/O devices.


Arithmetic Logic Unit (ALU): This is the fundamental building block of the CPU. It performs arithmetical and logical operations such as addition, subtraction, and Boolean functions (AND, OR, and NOT).

Control Unit: A digital circuit that controls CPU operations and coordinates the functionality of the CPU.


Memory and storage media are used to store data, memory modules enable data access at a higher speed than the storage media. Generally, there are two types of memory on a host:

Random Access Memory (RAM): This allows direct access to any memory location and can have data written into it or read from it. RAM is volatile; this type of memory requires a constant supply of power to maintain memory cell content. Data is erased when the system’s power is turned off or interrupted.

Read-Only Memory (ROM): Non-volatile and only allows data to be read from it. ROM holds data for execution of internal routines, such as system startup.

Storage devices are less expensive than semiconductor memory. Examples of storage devices are as follows:
·         Hard disk (magnetic)
·         CD-ROM or DVD-ROM (optical)
·         Floppy disk (magnetic)
·         Tape drive (magnetic)

I/O Devices

I/O devices enable sending and receiving data to and from a host. This communication may be one of the following types:

User to host communications: Handled by basic I/O devices, such as the keyboard, mouse, and monitor. These devices enable users to enter data and view the results of operations.

Host to host communications: Enabled using devices such as a Network Interface Card (NIC) or modem.

Host to storage device communications:  Handled by a Host Bus Adaptor (HBA). HBAs also provide connectivity outlets known as ports to connect the host to the storage device. A host may have multiple HBAs.

Connectivity: Connectivity refers to the interconnection between hosts or between a host and any other peripheral devices, such as printers or storage devices.

Physical components of connectivity

The three physical components of connectivity between the host and storage are Bus, Port, and Cable.

Physical components of connectivity

The bus is the collection of paths that facilitates data transmission from one part of a computer to another, such as from the CPU to the memory.


The storage device is the most important component in the storage system environment. A storage device uses magnetic or solid state media. Disks, tapes, and diskettes use magnetic media. CD-ROM is an example of a storage device that uses optical media, and removable flash memory card is an example of solid state media.

Tapes are a popular storage media used for backup because of their relatively low cost. In the past, data centers hosted a large number of tape drives and processed several thousand reels of tape.

Disk Drive Components

A disk drive uses a rapidly moving arm to read and write data across a flat platter coated with magnetic particles. Data is transferred from the magnetic platter through the R/W head to the computer.
Key components of a disk drive are platter, spindle, read/write head, actuator arm assembly, and controller.

Disk Drive Components

A typical HDD consists of one or more flat circular disks called platters. The data is recorded on these platters in binary codes (0s and 1s). The set of rotating platters is sealed in a case, called a Head Disk Assembly (HDA). A platter is a rigid, round disk coated with magnetic material on both surfaces (top and bottom).

Spindle and platter


A spindle connects all the platters and is connected to a motor. The motor of the spindle rotates with a constant speed. The disk platter spins at a speed of several thousands of revolutions per minute (rpm). Disk drives have spindle speeds of 7,200 rpm, 10,000 rpm, or 15,000 rpm.

Read/Write Head

Drives have two R/W heads per platter, one for each surface of the platter. The R/W head changes the magnetic polarization on the surface of the platter when writing data.

Actuator arm assembly

Actuator Arm Assembly

The R/W heads are mounted on the actuator arm assembly which positions the R/W head at the location on the platter where the data needs to be written or read.


The controller (see Figure 2-2 [b]) is a printed circuit board, mounted at the bottom of a disk drive. It consists of a microprocessor, internal memory, circuitry, and firmware. The firmware controls power to the spindle motor and the speed of the motor. It also manages communication between the drive and the host.

Physical Disk Structure

Data on the disk is recorded on tracks, Each track is divided into smaller units called sectors. A sector is the smallest, individually addressable unit of storage.

Disk structure: sectors, tracks, and cylinders

Disk Drive Performance

A disk drive is an electro-mechanical device that governs the overall performance of the storage system environment.

Various factors various factors that affect the performance of disk drives are discussed below.

Disk Service Time

Disk service time is the time taken by a disk to complete an I/O request. Components that contribute to service time on a disk drive are seek time, rotational latency, and data transfer rate.

Seek time

Time taken to position the R/W heads across the platter with a radial movement (moving along the radius of the platter).

Rotational Latency

The time taken by the platter to rotate and position the data under the R/W head is called rotational latency. Average rotational latency is around 5.5 ms for a 5,400-rpm drive, and around 2.0 ms for a 15,000-rpm drive.

Data Transfer Rate

The average amount of data per unit time that the drive can deliver to the HBA. It is important to first understand the process of read and write operations in order to calculate data transfer rates. In a read operation, the data first moves from disk platters to R/W heads, and then it moves to the drive’s internal buffer. Finally, data moves from the buffer through the interface to the host HBA. In a write operation, the data moves from the HBA to the internal buffer of the disk drive through the drive’s interface. The data then moves from the buffer to the R/W heads. Finally, it moves from the R/W heads to the platters.

Data transfer rate

Fundamental Laws Governing Disk Performance

To understand the laws of disk performance, a disk can be viewed as a black box consisting of two elements:

Queue: The location where an I/O request waits before it is processed by the I/O controller.

Disk I/O Controller: Processes I/Os that are waiting in the queue one by one.

I/O processing

The I/O requests arrive at the controller at the rate generated by the application. This rate is also called the arrival rate. These requests are held in the I/O queue, and the I/O controller processes them one by one. The I/O arrival rate, the queue length, and the time taken by the I/O controller to process each request determines the performance of the disk system, which is measured in terms of response time.

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January 11, 2016 at 5:56 PM delete

really good one bro....awesome explanation..

January 21, 2016 at 12:31 PM delete

Thanks Kavaskarg :) Please visit the site regularly for more updates.

February 1, 2016 at 2:04 PM delete

Thanks for useful info and nice explanation.