secondary storage structure - · pdf filemagnetic disks provide bulk of secondary storage of...

Secondary Storage Structure

By: Kaushik Vaghani

Prepared By: Kaushik Vaghani

Secondary-Storage Systems

• Overview of Mass Storage Structure

– Magnetic Disks

– Magnetic Tapes

• Disk Structure

• Disk Scheduling

• RAID Structure


Objectives

• Describe the physical structure of secondary and tertiary storage devices and the resulting effects on the uses of the devices

• Explain the performance characteristics of mass-storage devices

• Discuss operating-system services provided for mass storage, including RAID


Overview of Mass Storage Structure1. Magnetic disks provide bulk of secondary storage of modern computers

– Drives rotate at 60 to 200 times per second

– Transfer rate is rate at which data flow between drive and computer

– Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)

– Head crash results from disk head making contact with the disk surface

• That’s bad

• Disks can be removable

• Drive attached to computer via I/O bus

– Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI

– Host controller in computer uses bus to talk to disk controller built into drive or storage array


Moving-head Disk Mechanism


Overview of Mass Storage Structure (Cont.)

2. Magnetic tape

– Was early secondary-storage medium

– Relatively permanent and holds large quantities of data

– Access time slow

– Random access ~1000 times slower than disk

– Mainly used for backup, storage of infrequently-used data, transfer medium between systems

– Kept in spool and wound or rewound past read-write head

– Once data under head, transfer rates comparable to disk

– 20-200GB typical storage

– Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT


Overview of Mass Storage Structure (Cont.)

Magnetic tape


Disk Structure• Disk drives are addressed as large 1-dimensional arrays

of logical blocks, where the logical block is the smallest unit of transfer

• The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially

– Sector 0 is the first sector of the first track on the outermost cylinder

– Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost


Disk Attachment

• Host-attached storage accessed through I/O ports talking to I/O busses

• SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform tasks

– Each target can have up to 8 logical units (disks attached to device controller

• FC is high-speed serial architecture

– Can be switched fabric with 24-bit address space – the basis of storage area networks (SANs) in which many hosts attach to many storage units

– Can be arbitrated loop (FC-AL) of 126 devices


Network-Attached Storage• Network-attached storage (NAS) is storage made available over a

network rather than over a local connection (such as a bus)

• NFS and CIFS are common protocols

• Implemented via remote procedure calls (RPCs) between host and storage

• New iSCSI protocol uses IP network to carry the SCSI protocol


Storage Area Network• Common in large storage environments (and becoming

more common)

• Multiple hosts attached to multiple storage arrays -flexible


Disk Scheduling• The operating system is responsible for using hardware

efficiently — for the disk drives, this means having a fast accesstime and disk bandwidth

• Access time has three major components

– Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector

– Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head

– Transmission time is time to transfer information once it is under the head.

• For most disks, the seek time dominates the other two times, so reducing the mean seek time can improve system performance substantially. Prepared By: Kaushik Vaghani

Disk Scheduling (Cont.)• Seek time seek distance

• Disk bandwidth is the total number of bytes transferred, divided bythe total time between the first request for service and thecompletion of the last transfer

Types of disk arm scheduling algorithms:

1. FCSC (First come first serve)

2. SSTF (Shorted seek time first)

3. SCAN (Elevator)

4. C-SCAN

5. LOOK

6. C-LOOKPrepared By: Kaushik Vaghani

Disk Scheduling (Cont.)

• Several algorithms exist to schedule the servicing of disk I/O requests

• We illustrate them with a request queue (0-199)

98, 183, 37, 122, 14, 124, 65, 67

Head pointer 53


FCFS (First come first serve)• Here requests are served in the order of their arrival.

• Illustration shows total head movement of 640 cylinders

• (98-53)+(183-98)+(183-37)+(122-37)+(122-14)+(124-14)+(124-65)+(67-65) = 640 Prepared By: Kaushik Vaghani

SSTF (Shortest seek time first)• Closest cylinder next Scheduling

• Selects the request with the minimum seek time fromthe current head position

• SSTF scheduling is a form of SJF scheduling; may causestarvation of some requests

• Illustration shows total head movement of 236cylinders


SSTF (Cont.)

• (65-53)+(67-65)+(67-37)+(37-14)+(98-14)+(122-98)+(124-122)+(183-124) = 236 Prepared By: Kaushik Vaghani

SCAN

• The disk arm starts at one end of the disk, and movestoward the other end, servicing requests until it getsto the other end of the disk, where the headmovement is reversed and servicing continues.

• SCAN algorithm Sometimes called the elevatoralgorithm

• Illustration shows total head movement of 208cylinders


SCAN (Cont.)


C-SCAN

• Circular-SCAN Scheduling

• Provides a more uniform wait time than SCAN

• The head moves from one end of the disk to the other,servicing requests as it goes

– When it reaches the other end, however, itimmediately returns to the beginning of the disk,without servicing any requests on the return trip

• Treats disk as if it were circular with the last trackadjacent to the first one

• Always goes in one directionPrepared By: Kaushik Vaghani

C-SCAN (Cont.)


LOOK

• Version of SCAN

• Arm only goes as far as the last request in eachdirection, then reverses direction immediately,without first going all the way to the end of thedisk.

• Keep moving in the same direction until there are no more outstanding requests pending in that direction, then algorithm switches the direction.

• After switching the direction the arm will move to handle any request on the way.


LOOK

As per LOOK request will be satisfied in order 11, 12, 16, 34, 36, 9, 1.Total cylinder movement: (12-11) + (16-12) + (34-16) + (36-34) + (36-9) + (9-1)=60

disk with 51 cylinders

Current head position : 111, 36, 16, 34, 9, and 12


C-LOOK

• Version of C-SCAN

• Arm only goes as far as the last request ineach direction, then reverses directionimmediately, without first going all the wayto the end of the disk


C-LOOK (Cont.)


Example:

Consider an imaginary disk with 51 cylinders. A request comes in to read ablock on cylinder 11. While the seek to cylinder 11 is in progress, newrequests come in for cylinders 1, 36, 16, 34, 9, and 12, in that order.

Starting from the current head position, what is the total distance (incylinders) that the disk arm moves to satisfy all the pending requests, for eachof the following disk scheduling Algorithms?

1. FCSC (First come first serve)

2. SSTF (Shorted seek time first)

3. SCAN

4. C-SCAN

5. LOOK

6. C-LOOK


FCSC (First come first serve) Scheduling

Here requests are served in the order of their arrival.

In given example disk movement will be 11, 1, 36, 16, 34, 9 and 12 as first come firstserved.

Total cylinder movement: (11-1)+ (36-1)+ (36-16) + (34-16) +(34-9)+(12-9) = 111

cylinders 1, 36, 16, 34, 9, and 12


SSTF (Shortest seek time first)

We can minimize the disk movement by serving the request closest to the currentposition of the head.

In given example starting head position is 11, closest to 11 is 12, closest to 12 is 9, andso on.

As per SSTF request will be satisfied in order 11, 12, 9, 16, 1, 34, 36.

Total cylinder movement: (12-11) + (12-9) + (16-9) + (16-1) + (34-1) + (36-34) = 61

cylinders 1, 36, 16, 34, 9, and 12


LOOK disk arm scheduling

As per LOOK request will be satisfied in order 11, 12, 16, 34, 36, 9, 1.

Total cylinder movement: (12-11) + (16-12) + (34-16) + (36-34) + (36-9) + (9-1)=60

cylinders 1, 36, 16, 34, 9, and 12


C-LOOK

As per CLOOK requests will be satisfied in order 11, 12, 16, 34, 36, 1, 9

Total cylinder movement: (12-11) + (16-12) + (34-16) + (36-34) +(36-1)+(9-1)=68

cylinders 1, 36, 16, 34, 9, and 12


SCAN

As per SCAN request will be satisfied in order: 11, 12, 16, 34, 36, 50, 9, 1

Total cylinder movement: (12-11) + (16-12) + (34-16) +(36-34) +(50-36) + (50-9) + (9-1)= 88

cylinders 1, 36, 16, 34, 9, and 12


CSCAN

As per SCAN request will be satisfied in order: 11, 12, 16, 34, 36, 50, 0, 1,9

Total cylinder movement: (12-11) + (16-12) + (34-16) +(36-34) +(50-36) + (50-0) + (1-0)+(9-1) = 98

cylinders 1, 36, 16, 34, 9, and 12


Selecting a Disk-Scheduling Algorithm

• SSTF is common and has a natural appeal

• SCAN and C-SCAN perform better for systems that place a heavyload on the disk

• Performance depends on the number and types of requests

• Requests for disk service can be influenced by the file-allocationmethod

• The disk-scheduling algorithm should be written as a separatemodule of the operating system, allowing it to be replaced witha different algorithm if necessary

• Either SSTF or LOOK is a reasonable choice for the defaultalgorithm


RAID Structure• Disk drives have continued to get smaller and cheaper, so it is

now economically feasible to attach many disks to a computersystem.

• Having a large number of disks in a system presentsopportunities for improving the rate at which data can be reador written, if the disks are operated in parallel.

• Furthermore, this setup offers the potential for improving thereliability of data storage, because redundant information canbe stored on multiple disks. Thus, failure of one disk does notlead to loss of data.

• A variety of disk-organization techniques, collectively calledredundant array of independent disks (RAIDs), are commonlyused to address the performance and reliability issues.Prepared By: Kaushik Vaghani

RAID Structure• RAID – multiple disk drives provides reliability via

redundancy

• All RAID have the property that the data are distributedover drives, to allow parallel operation.

• RAID is arranged into six different levels


RAID (Cont.)• Several improvements in disk-use techniques involve the use of

multiple disks working cooperatively

• Disk striping uses a group of disks as one storage unit

• RAID schemes improve performance and improve the reliability ofthe storage system by storing redundant data

– Mirroring or shadowing (RAID 1) keeps duplicate of each disk

– Striped mirrors (RAID 1+0) or mirrored stripes (RAID 0+1)provides high performance and high reliability

– Block interleaved parity (RAID 4, 5, 6) uses much lessredundancy


RAID Levels


RAID (0 + 1) and (1 + 0)


Thank You


secondary storage structure - · pdf filemagnetic disks provide bulk of secondary storage of...

Documents