CS307 Operating Systems

Mass-Storage Systems

Fan WuDepartment of Computer Science and Engineering

Shanghai Jiao Tong University

Fall 2011

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Chapter 12: Mass-Storage Systems

Overview of Mass Storage Structure

Disk Structure

Disk Attachment

Disk Scheduling

Disk Management

RAID Structure

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Objectives

Describe the physical structure of secondary 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

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Moving-head Disk Mechanism

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The First Commercial Disk Drive

1956IBM RAMDAC computer included the IBM Model 350 disk storage system

5M (7 bit) characters50 x 24” plattersAccess time = < 1 second

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Modern Disk Drive

A Western Digital 3.5 inch 250 GB SATA HDD

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Different Sized Disk Drives

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Read-Write Arm and Head

Head stack Read-write head

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Overview of Mass Storage Structure

Magnetic disks provide bulk of secondary storage of modern computers

Drives rotate at 60 to 250 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)

Drive attached to computer via I/O bus

Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI, SAS, Firewire

Host controller in computer uses bus to talk to disk controller built into drive

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Magnetic Tape

Was early secondary-storage medium

Evolved from open spools to cartridges

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

140MB/sec and greater

200GB to 1.5TB typical storage

Common technologies are LTO-{3,4,5} and T10000

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Solid-State Drive (SSD)

SSDs use microchips which retain data in non-volatile memory chips and contain no moving parts

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Disk Structure

cylindersector

block

0

1

2

34

5

6

7

8

9

10

1112

13

14

15

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ……

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Disk Structure (Cont.)

Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer

512 bytes logical block size

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

Difficulty in mapping from logical to physical address:

Except for bad sectors

Non-constant # of sectors per track via constant angular velocity

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Disk Scheduling (Cont.) There are many sources of disk I/O request

OS, System processes, Users processes

OS maintains queue of requests, per disk or device

Idle disk can immediately work on I/O request, busy disk means work must queue

Optimization algorithms only make sense when a queue exists

The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth

Minimize seek time seek distance

What about rotational latency?

Difficult for OS to calculate

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

We illustrate scheduling algorithms with a request queue (0-199)

98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53

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Disk-Scheduling Algorithms

First-Come, First-Served (FCFS) Scheduling

Shortest Seek Time First (SSTF) Scheduling

SCAN Scheduling

C-SCAN Scheduling

LOOK/C-LOOK Scheduling

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FCFS Scheduling

Total head movement: 640 cylinders

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SSTF Scheduling Shortest Seek Time First selects the request with the minimum seek time

from the current head position

Total head movement: 236 cylinders.

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SCAN The disk arm starts at one end of the disk, and moves toward the other end,

servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.

Total head movement: 236 cylinders

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C-SCAN The head moves from one end of the disk to the other, servicing requests as

it goes. When it reaches the other end, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.

Total head movement: 382 cylinders

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C-LOOK Arm only goes as far as the last request in each direction, then reverses

direction immediately, without first going all the way to the end of the disk

Total head movement: 354 cylinders

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Test Yourself

Suppose that a disk drive has 5,000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 143, and the previous request was at cylinder 125. The queue of pending requests, in FIFO order, is:

86,1470,913,1774,948,1509,1022,1750,130

Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests for each of the following disk-scheduling algorithms?

a) SSTF

b) C-LOOK

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Selecting a Disk-Scheduling Algorithm

SSTF is common and has a natural appeal

SCAN and C-SCAN perform better for systems that place a heavy load on the disk

Less starvation

LOOK and C-LOOK have a little improvement over SCAN and C-SCAN

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RAID Structure

Redundant Arrays of Inexpensive Disks (RAIDs)

RAID – multiple disk drives provides reliability via redundancy

Mirroring

duplicate every disk

Parity bit

Parallel access to multiple disk improves performance

Bit-level striping

split the bits of each byte across multiple disks

block-level striping

blocks of a file are striped across multiple disks

RAID is arranged into seven different levels

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RAID Levels

C: a second copy

P: error-correcting bit

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RAID Levels (Cont.)

Block Striping

Striped Mirroring

Memory-style Error-Correcting Code (ECC)

P1 P2 P3

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RAID Levels (Cont.)

bit-interleaved parity

block-interleaved parity

block-interleaved distributed parity

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RAID Levels (Cont.)

RAID 6: P + Q redundancy

Reed-Solomon codes

2 bits of redundant data are stored for every 4 bits of data

can tolerate two disk failures

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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

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Network-Attached Storage

Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus)

Remotely attaching to file systems

NFS and CIFS are common protocols

Implemented via remote procedure calls (RPCs) between host and storage over typically TCP or UDP on IP network

iSCSI protocol uses IP network to carry the SCSI protocol

Remotely attaching to devices (blocks)

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Storage Area Network

Common in large storage environments

Multiple hosts attached to multiple storage arrays - flexible

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Homework

Reading

Chapter 12