8/7/2019 Unit 2san

1/27

Unit 2

SANDEEP KHANDEKAR,

ASSISTANT PROFESSOR,

SNIST

HYDERABAD

8/7/2019 Unit 2san

2/27

Syllabus Hardware and software components of the host environment.

Key protocols and concepts used by each component.

Physical and logical components of a connectivity environment.

Major physical components of a disk drive and their function.

Logical constructs of a physical disk.

Access characteristics and performance implications.

Concept of RAID and its components.

Different RAID levels and their suitability for different application environments :

RAID 0, RAID1, RAID 2, RAID 3, RAID 4, RAID 5, RAID 0+1, RAID 1+0, RAID 6.

Compare and contrast integrated and modular storage systems.

High level architecture and working of an intelligent storage system.

8/7/2019 Unit 2san

3/27

INTRODUCTION

Data flows from an application to storage through

8/7/2019 Unit 2san

4/27

8/7/2019 Unit 2san

5/27

8/7/2019 Unit 2san

6/27

8/7/2019 Unit 2san

7/27

8/7/2019 Unit 2san

8/27

8/7/2019 Unit 2san

9/27

Host BUS Adapter

Host BUS Adapter

8/7/2019 Unit 2san

10/27

Disk Drive Performance

A disk drive is an electromechanical device that governs the overall

performance of the storage system environment.

Various factors effect the disk drive performance.

Disk service Time: Time taken by disk to complete an I/O request.

Components that contribute to disk service time on a disk drive

are seek time, rotational latency, and data transfer rate.

8/7/2019 Unit 2san

11/27

Seek Time

Time taken to position the Read/Write heads across the platter with

a radial movement (Moving along the radius of the platter).

Time taken to reposition and settle the arm and the head over the

correct track.

The lower the seek time, the faster the I/O operation.

8/7/2019 Unit 2san

12/27

Disk vendors publish the following seek time specifications

Full Stroke: The time taken by the R/W head to move across the

entire width of the disk. From the innermost track to the outermost

track.

Average: The average time taken by the R/W head to move from

one random track to another, normally listed as the time for one third

of a full stroke.

Track to Track : The time taken by the R/W head to move between

adjacent tracks.

8/7/2019 Unit 2san

13/27

Each of these specifications is measured in milliseconds.

The average seek time of the modern disk is typically in the range of 3 to 15

milliseconds.

Seek time has more impact on the read operation ofrandom tracks rather than

adjacent tracks.

To minimize the seek time, data can be written to only a subset of the available

cylinders. This results in lower useable capacity than the actual capacity of the

drive.

Example : A 500GB disk drive is set up to use only the 40% of the cylinders and

is effectively treated as a 200GB drive. This is known as short stroking the

drive.

8/7/2019 Unit 2san

14/27

Rotational Latency To access data, the actuator arm moves the R/W head over the platter to a

particular track while the platter spins to position the requested sector underthe R/W head.

The time taken by the platter to rotate and position the data under the R/Whead is called rotational latency.

This latency depends on the rotation speed of the spindle and is measured inmilliseconds.

The average rotational latency is one-half of the time taken for a full rotation.

Rotational latency has more impact on the reading/writing of random sectors onthe disk than on the same operations on the adjacent sectors.

Average rotational latency is around 5.5ms for a 5,400rpm drive, and around2.0ms for a 15,00rpm drive.

8/7/2019 Unit 2san

15/27

Data Transfer rate The data transfer rate refers to the average amount of data per unit time that

the drive can deliver to the HBA.

Operations in order to calculate data transfer rates : Read, write operations

Read Operation: Data first moves from disk platters to R/W heads, and then it

moves to the drives internal buffer. Finally data moves from the buffer to the

host HBA.

Write Operation: Data moves from the HBA to the internal buffer of the disk

drive through the drives interface. The data then moves from the buffer to the

R/W heads. Finally it moves from the R/W heads to the platters.

8/7/2019 Unit 2san

16/27

HBAHead Disk

AssemblyInterface

Buffer

Controller

External transfer rate

measured here

Internal transfer rate

measured here

Figure : Data Transfer rate

Disk

8/7/2019 Unit 2san

17/27

Internal Transfer rate: Speed at which data moves from a single track of a

platters surface to internal buffer (cache) of the disk.

Internal transfer rate takes into account factors such as the seek time.

External Transfer rate: Rate at which data can be moved through theinterface to the HBA.

External transfer rate is generally the advertised speed of the interface,

such as 133MB/s for ATA.

The sustained external transfer rate is lower than the interface speed.

8/7/2019 Unit 2san

18/27

Fundamental Laws Governing Disk

Performance Disk can be viewed as a black box consisting of two elements.

1. Queue : The location where an I/O request waits before it is

processed by the I/O Controller.

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

one by one.

8/7/2019 Unit 2san

19/27

Figure : I/O Processing

123456 I/O Controller Processed

I/ORequest

Arrival

I/O Queue

Arrival Rate: The I/O request arriving at the controller at the rate

generated by the application.

Response Time: I/O arrival rate, the queue length, and the time

taken by the I/O controller to process each request determines the

performance, measured in terms of response time

8/7/2019 Unit 2san

20/27

Laws

Little`s Law: Describes the relationship between the number of requests in a queue

and the response time.

N=a X R ---------- eq(1)

N= Total Number of requests in the queuing system (request in the queue + request

in the I/O controller)

a= arrival rate or the number of I/O requests that arrive to the system per unit of time.

R= average response time or the turn around time for an I/O request the total time

from the arrival to departure from the system.

8/7/2019 Unit 2san

21/27

Utilization Law: Defines the I/O controller utilization

U=a X Rs ----------------------------------eq(2)

U is the I/O controller utilization.

Rs is the service time, average time spent by a request on the controller. (1/

Rs is the service rate)

From the arrival rate a , the average inter-arrival time Ra , can be computed

as

Ra =1/a----------------------------------eq(3)

8/7/2019 Unit 2san

22/27

U=Rs/Ra ----------------------------------eq(4)

The value of this ratio varies between 0 and 1.

Calculating : average response time, average queue length, time spent by

a request in a queue.

Average Response rate (S).

S=service rate(1/Rs) arrival rate(1/Ra).

Average response rate can be defined as the reciprocal of the average

response time (R).

R=1/S

8/7/2019 Unit 2san

23/27

R=Rs/(1-U) --------------eq(5)

Number of requests in the queue (NQ) = Number of Requests in the

system (N) Number of Requests on the controller or utilization (U)

NQ = N-U.

Time spent by a request in the queue is equivalent to the time spent by arequest in the system, or the average response time minus the time spent

by a request on the controller for processing.

Time Spent = Avg. Response Time(R) Time Spent by a request(Rs).

8/7/2019 Unit 2san

24/27

Low Queue Size

Knee of curve: Disks at

about 70% utilization

Figure : Utilization Vs Response Time

8/7/2019 Unit 2san

25/27

Logical Components of the Host

Operating System.

Device drivers.

Volume Manager. File System.

Application.

8/7/2019 Unit 2san

26/27

File System File system is a hierarchical structure of files.

File system enables easy access to data files residing within a disk drive, a

disk partition or a logical volume.

A file system needs host-based logical structures and software routines thatcontrol access to files.

A file system organizes data in a structured hierarchical manner via the use

of directories, which are containers for storing pointers to multiple files,

All file systems maintain a pointer map to the directories, subdirectories, and

files that are part of the file system.

8/7/2019 Unit 2san

27/27