Chapter 11 hard disk

7/27/2019 Chapter 11 hard disk

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Chapter 11:

Mass-Storage Structure

Disk attachment

SSD (Solid state drive) vs. HDD (Hard diskdrive)

Disk I/O scheduling

Disk management

Formatting/partitioning


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Disk Attachment

Host-attached storage Storage accessed on host through local I/O port

Uses hardware bus and host controller (e.g., IDE, ATA,

SATA, FireWire, USB, SCSI, FCFiber Channel)

Network-attached storage Special purpose storage system attached remotely over

a data network

Clients access severs via RPC (e.g., NFS for Unix

systems or CIFS for Windows systems)

Storage-area network

Private network (using storage protocols) connecting

file servers and storage units2


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Erase and SSD

Erase: Writing to the SSD must be

preceded by a block erase (sets all bits to 1)

Lifetime: Measured in the number of erase

cycles

Wear leveling: Firmware or operating

system drivers must balance the numbers oferase cycles done on each block so that

device does not fail prematurely

4


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SSD Performance

Single chip has relatively high latencies(SLC NAND)

~25 s to fetch (read) a 4K page from the array

to the IO buffer

~250 s to commit (write) a 4K page from the

IO buffer to the array

~2 ms to erase a 256 kiB block

With parallel chip operation, 250 MB/seffective read/write speeds

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http://en.wikipedia.org/wiki/Solid-state_drive
http://en.wikipedia.org/wiki/Solid-state_drivehttp://en.wikipedia.org/wiki/Solid-state_drivehttp://en.wikipedia.org/wiki/Solid-state_drivehttp://en.wikipedia.org/wiki/Solid-state_drive


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hdparm command

Measuring disk performance yourself on

Linux

http://www.cyberciti.biz/tips/how-fast-is-linux-sata-hard-disk.html

Old disk system: 57.9 MB/sec

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Virtual Box Linux

sudo hdparm -tT /dev/sda

/dev/sda:

Timing cached reads: 15392 MB in 2.00

seconds = 7705.18 MB/sec

Timing buffered disk reads: 154 MB in 3.02

seconds = 50.96 MB/sec

7


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8Figure 11.1 Moving-head disk mechanism (HDD)


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http://www.cprince.com/courses/cs5631spring09/news/item1.pdf

(Presentation by Yizhao Zhuang)


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HDD (Hard Disk Drive) Terms Access (positioning) time

Time to start to transfer dataComponents: Seek time, rotational latency

Seek time: time for disk arm to move heads to theright cylinder

Rotational latency: time for disk to rotate to thedesired sector

Transfer rate

Sustained bandwidth: average data transfer

rate during a large transferthat is the, numberof bytes divided by transfer time

data rate without positioning time

Effective bandwidth: average transfer rate

including positioning time


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HDD Scheduling

Modern disks are accessed as a large arrayof blocks:

a disk addressis a block number

Block number is converted to a old styledisk address (i.e., cylinder, head, sector) bythe disk device firmware

Generally, increasing block number impliesphysically adjacent sectors (see alsohttp://www.linuxjournal.com/article/6931),and movement to inner cylinders of disk
http://www.linuxjournal.com/article/6931http://www.linuxjournal.com/article/6931


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Some Algorithms To Reduce Seek Time

Algorithm Name Description

FCFS First-come first-served

SSTF Shortest seek time first; process the

request that reduces next seek time

SCAN (akaElevator)

Move head from end to end (has acurrent direction)

C-SCAN Only service requests in one direction

(circular SCAN)

LOOK Move head from end to end; turn atlast request in direction

C-LOOK Only service requests in one direction

and turn at last request in direction

(circular LOOK)


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Scheduling Examples

Assume List (queue) of track requests:

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

Left most is first request

R/W head starts at track 53

Tracks range from 0-199

Only a single platter

Question:Given a specific scheduling algorithm what is

the total head movement required for a list oftrack requests?


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FCFS

Assume

List (queue) of track requests:

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





1) Show the head movement over the disk surface2) What is the total head movement?


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FCFS

Total head movement: 640 tracks

98-53=45

183-98=85

183-37=146122-37=85

122-14=108

124-14=110

124-65=59

67-65=2


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FCFS Performance Issues

It is possible for disk positions far from a

current area of activity to bestarved

indefinitely?

No. With FCFS, incoming (new) requests

will eventually get processed.

We are not reordering requests here.

The algorithm is intrinsically fair, but it

generally does not generally provide the

fastest service.


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SSTF: Shortest Seek Time First

Assume


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





1) Show the head movement over the disk surface2) What is the total head movement?


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SSTF exampleQueue Head pos Seek time

98 53 45

183 53 130

37 53 16

122 53 69

14 53 39

124 53 71

65 53 12

67 53 14

SST= 12

Queue Head pos Seek time

98 65 33

183 65 118

37 65 28

122 65 57

14 65 51

124 65 59

67 65 2

SST= 2


98 67 31

183 67 116

37 67 30

122 67 55

14 67 53

124 67 57

SST= 30


98 37 61

183 37 146

122 37 85

14 37 23

124 37 87

SST= 23


98 14 84

183 14 169

122 14 108

124 14 110

SST= 84


183 98 85

122 98 24

124 98 26

SST= 24


183 122 61

124 122 2

SST= 2


183 124 59

SST= 59

Total head movement:

236 tracks


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SSTF

Total head movement:236 tracks


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SSTF Performance Issues

It is possible for disk positions far from a current

area of activity to be starved indefinitely?

YES: If new requests keep arriving for the current area

Prefer a method that we know will not starve

requests

Also, while better than FCFS (236 for SSTF vs. 640

for FCFS), less head movement can be obtained E.g., service order: 53, 37, 14, 65, 67, 98, 122, 124, 183

has total head movement of 208 tracks

SCAN


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SCAN

Algorithm: Move head from end to end (has a

current direction), servicing requests as you cometo them

Assume


98, 183, 37, 122, 14, 124, 65, 67 Left most is first request




Start direction = down (to lower numbered tracks)

1) Show the head movement over the disk surface

2) What is the total head movement?

From start position to servicing last request in list


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SCAN

Total head movement: 53 + 183 = 236


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SCAN Performance Issues

It is possible for disk positions far from a

current area of activity to be starved

indefinitely?


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C-SCAN

Algorithm: Only service requests in one direction(circular SCAN)

Assume


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





Start direction = up (to higher numbered tracks) Service requests (only) in up direction

1) Show the head movement over the disk surface

2) What is the total head movement?


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C-LOOK

Total head movement: (183-53)+(37-14)=153 (+ wrap)


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Algorithm Selection Issues

Starvation FCFS, SCAN and LOOK algorithms will not starve

requests

SSTF can starve requests

Type of request activity Some requests will be to similar tracks of disk

E.g., Requests by a single process for a file allocated using

contiguous file allocation

FCFS, SSTF: Should do well with these clustered requests Some requests will be on widely different tracks of disk

E.g., Indexed, linked file allocation; requests by different

processes

May be best to have algorithm that distributes requests

uniformly across surface of disk (e.g., SCAN, LOOK)


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Ch 11.5: Disk Management

Disk formatting

Swap space

Boot blocks & Booting


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Ch 11.5.1: Disk Formatting Three steps

Low-level formatting, partitioning, logical formatting

Low-level formatting

Fills disk sectors with special data structure that is usedby disk I/O controller

Header, data area (usually 512 bytes), trailer Header & trailer: information used by disk controller

hardware-- e.g., sector number and ECC (error correctingcode)

Creates map of bad blocks

Reserves spare sectors for bad block repair

Partitioning

Grouping disk into one or more groups of cylinders tobe treated as separate logical disks

Each partition can have its own file system type


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Logical Formatting

(Per Partition)

Creation of organization for different types of filesystems, swap space, database application etc. E.g., directory structures, free space lists

Boot partitions Boot block; code to load kernel of O/S

It is also possible to leave the partition withoutlogical formatting With no file system disk data structures

Partition accessed as an array of blocks raw disk, and raw I/O

Raw I/O bypasses file system services such as buffercache, file locking, prefetching, space allocation, filenames, and directories

H dli B d Bl k


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Handling Bad Blocks Can handle at free blocks level in O/S

Free list of blocks can be used to indicate these blockscannot be used

Device controller

Can maintain list of bad blocks

Initialized during low-level formatting at factory

Low-level formatting also sets aside spare sectors, notvisible to O/S

Sectorsparing or forwardingcan be used to managenew bad blocks

Sparing or forwarding: Controller uses a spare sector when itgets a request for a sector that has a bad block

When block is remapped, controller uses a spare sector fromthe same cylinder if possible

Sector slipping: Moving a sequence of blocks & re-

mapping them

S ap Space


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Swap Space Virtual memory

Will use regular file system for at least reading code

and data of program Swap space may be on separate device or multiple

devices

Partition formatted for swap space can give highervirtual memory performance: Why?

With a separate partition, swap space managerallocates/deallocates blocks

Doesnt use functions of normal file system

Maintain map(s) of swap space usage

May handle text (code) & data differently

E.g., Solaris 1dont write text to swap disk; if pagereplacement needs page, next time just re-read fromnormal file system; dirty data pages still written to swap

disk as needed


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Boot Process

Typically a three phase process ROM bootstrap Boot block loader

Running operating system kernel

ROM bootstrap program Initial program code that starts loading operating

system

Typically a relatively simple loader program stored in

ROM;reads the disk boot block(s) into RAM, andstarts running this boot block code

Boot block code

Actually does loading of operating system kernel fromboot partition

RAID: Redundant Array of


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RAID: Redundant Array of

Independent/Inexpensive Disks

Multiple disk techniques for Improving throughput and/or response time

Multiple disks, parallelize disk operations

Improving data reliability

Decrease mean time to failure (MTBF) If one disk fails, can replace with another and go on quickly,

with easy recovery

Ideas Data striping: Methods of improving throughput

Putting part of data on disk A, part on disk B, part on

Bit striping: e.g., 8 disks, one bit of each byte on each disk

Block striping: n disks, blocki goes on disk (i mod n) + 1

Methods of improving reliability

Mirroring(a.k.a.shadowing): duplicating all data

Parity, ECC (error correcting codes)


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Resources

Kernel Korner - I/O Schedulers

http://www.linuxjournal.com/article/6931

Describes soft deadline disk head schedulingand anticipatory scheduling in Linux 2.6 kernel

See also the CFQ scheduler (Complete Fair

Queuing Scheduler)http://en.wikipedia.org/wiki/CFQ
http://www.linuxjournal.com/article/6931http://www.linuxjournal.com/article/6931


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END!

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Chapter 11 hard disk

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