oses: 13. secondary storage 1 operating systems v objectives –introduce issues such as disk...

OSes: 13. Secondary Storage 1

Operating SystemsOperating Systems

ObjectivesObjectives– introduce issues such as disk scheduling, introduce issues such as disk scheduling,

formatting, and swap space managementformatting, and swap space management

Certificate Program in Software DevelopmentCSE-TC and CSIM, AITSeptember--November, 2003

13. Secondary Storage(S&G, Ch. 13)


ContentsContents

1.1. A Hard Disk (Again)A Hard Disk (Again)

2.2. Disk SchedulingDisk Scheduling

3.3. Disk FormattingDisk Formatting

4.4. Boot BlocksBoot Blocks

5.5. Swap Space ManagementSwap Space Management

6.6. Disk ReliabilityDisk Reliability


1. A Hard Disk (Again)1. A Hard Disk (Again) Fig. 2.5, p.33

:

rotation

armplatter

cylinder c

track t

read-writehead

spindleactuator

sector s

continued


The Logical View:The Logical View:– one-dimensional array of logical one-dimensional array of logical blocksblocks– a block is the smallest transfer unita block is the smallest transfer unit

(typically 512-1024 bytes)(typically 512-1024 bytes)

Mapping to the hardware:Mapping to the hardware:– for each cylinder (outer to inner)for each cylinder (outer to inner)

start at first sector of outermost trackstart at first sector of outermost track go round the trackgo round the track move to the next track on the cylindermove to the next track on the cylinder


Mapping IssuesMapping Issues

Avoiding defective sectors:Avoiding defective sectors:– use substitutes from elsewhere on the diskuse substitutes from elsewhere on the disk

The number of sectors varies per trackThe number of sectors varies per track– less sectors per track in the centreless sectors per track in the centre

ZonesZones– a zone is a collection of tracks whose a zone is a collection of tracks whose

sectors/track are the samesectors/track are the same


2. Disk Scheduling2. Disk Scheduling Fast access times requires:Fast access times requires:

– fast seek timefast seek time move head to the right cylinder quicklymove head to the right cylinder quickly

– low rotational latencylow rotational latency rotate the disk under the head quickly so the rotate the disk under the head quickly so the

required sector can be accessedrequired sector can be accessed

High disk bandwidthHigh disk bandwidth– total number of bytes transferred in the time total number of bytes transferred in the time

between the initial request and its completionbetween the initial request and its completion


Disk I/O requests are placed on a queue Disk I/O requests are placed on a queue managed by the hard drive controller.managed by the hard drive controller.

When the controller comes to do the next When the controller comes to do the next request, its scheduler chooses a request request, its scheduler chooses a request which:which:– improves access timeimproves access time– improves disk bandwidthimproves disk bandwidth

Controller SelectionController Selection


Types of Disk SchedulingTypes of Disk Scheduling

First-Come First-Served (FCFS)First-Come First-Served (FCFS)

Shortest Seek-Time First (SSTF)Shortest Seek-Time First (SSTF)

SCAN (and C-SCAN)SCAN (and C-SCAN)

LOOK (and C-LOOK)LOOK (and C-LOOK)


2.1. FCFS Scheduling2.1. FCFS Scheduling

FairFair Not the fastest serviceNot the fastest service Large swings across the disk are possible.Large swings across the disk are possible.

The following example(s) will assume a The following example(s) will assume a disk queue of I/O requests to blocks on disk queue of I/O requests to blocks on cylinders.cylinders.


ExampleExample

Queue: 98, 183, 37, 122, 14, 124, 65, 67Queue: 98, 183, 37, 122, 14, 124, 65, 67 Head starts at cylinder 53.Head starts at cylinder 53.

Fig. 13.1, p.433

14 37 53 65 67 98 122 124 183

Total head movement = 640 cylinders


2.2. SSTF Scheduling2.2. SSTF Scheduling

The next request to be serviced is the one The next request to be serviced is the one closest to the current head positionclosest to the current head position– minimize seek timeminimize seek time

Reduces overall head movement.Reduces overall head movement.

May lead to May lead to starvationstarvation for some requests. for some requests.


ExampleExample

Queue: 98, 183, 37, 122, 14, 124, 65, 67Queue: 98, 183, 37, 122, 14, 124, 65, 67 Head starts at cylinder 53.Head starts at cylinder 53.

Fig. 13.2, p.434


14 37 53 65 67 98 122 124 183


Not OptimalNot Optimal

If the queue was serviced in a slightly If the queue was serviced in a slightly different order:different order:– 53,53, 37, 14 37, 14, 65, 67, 98, 122, 124, 183, 65, 67, 98, 122, 124, 183

– then the total head movement would be less: then the total head movement would be less: 208208 cylinders cylinders


2.3. SCAN Scheduling2.3. SCAN Scheduling

Scan back and forth across the diskScan back and forth across the disk– sometimes called the sometimes called the elevatorelevator algorithm algorithm

ProblemProblem: if a request arrives just behind the : if a request arrives just behind the head then it will have to wait until the head head then it will have to wait until the head returns.returns.


ExampleExample

Queue: 98, 183, 37, 122, 14, 124, 65, 67Queue: 98, 183, 37, 122, 14, 124, 65, 67 Head starts at cylinder 53, Head starts at cylinder 53, andand moves left. moves left.

Fig. 13.3, p.435


14 37 53 65 67 98 122 124 183


C-SCAN SchedulingC-SCAN Scheduling

‘‘C’ for “circular list”.C’ for “circular list”. When a scan reaches one end of the disk, it When a scan reaches one end of the disk, it

jumps to the other endjumps to the other end– no point reversing since it is unlikely that there no point reversing since it is unlikely that there

will be requests in recently serviced areaswill be requests in recently serviced areas


ExampleExample

Queue: 98, 183, 37, 122, 14, 124, 65, 67Queue: 98, 183, 37, 122, 14, 124, 65, 67 Head starts at cylinder 53, Head starts at cylinder 53, andand moves right. moves right.

Fig. 13.4, p.436

14 37 53 65 67 98 122 124 183


2.4. LOOK Scheduling2.4. LOOK Scheduling

SCAN (C-SCAN) moves the head across SCAN (C-SCAN) moves the head across the full width of the disk.the full width of the disk.

LOOK (C-LOOK) moves the head only as LOOK (C-LOOK) moves the head only as far as the last request in each direction, and far as the last request in each direction, and then reverses (wraps around).then reverses (wraps around).


C-LOOK ExampleC-LOOK Example

Queue: 98, 183, 37, 122, 14, 124, 65, 67Queue: 98, 183, 37, 122, 14, 124, 65, 67 Head starts at cylinder 53, Head starts at cylinder 53, andand moving right. moving right.

Fig. 13.5, p.437

14 37 53 65 67 98 122 124 183


2.5. Which Disk Scheduling Algorithm?2.5. Which Disk Scheduling Algorithm?

SSTF - commonly usedSSTF - commonly used SCAN/C-SCANSCAN/C-SCAN

– good for heavily loaded disks since they avoid good for heavily loaded disks since they avoid starvationstarvation

Choice depends on number/type of Choice depends on number/type of requestsrequests– e.g. file I/O will generate sequences of e.g. file I/O will generate sequences of

requests to adjacent blocksrequests to adjacent blocks

continued


Caching?Caching?

Comparisons can utilise seek distances (as Comparisons can utilise seek distances (as here) here) andand disk rotational latency. disk rotational latency.

Logical addresses hide useful physical Logical addresses hide useful physical information (e.g. sector and track positions), information (e.g. sector and track positions), that an OS-level scheduler could use.that an OS-level scheduler could use.

continued


There may be conflict between the OS and There may be conflict between the OS and disk controller scheduling policies:disk controller scheduling policies:– e.g. the OS may want disk writes to happen e.g. the OS may want disk writes to happen

immediately and in a fixed order when a immediately and in a fixed order when a transaction is being carried outtransaction is being carried out


3. Disk Formatting3. Disk Formatting

The disk controller uses The disk controller uses low-level low-level formattingformatting (physical formatting) (physical formatting)– a data structure per each sectora data structure per each sector– consists ofconsists of

header, data area (512 bytes), trailerheader, data area (512 bytes), trailer

– utilises an error-correcting code (ECC)utilises an error-correcting code (ECC)

continued


The OS adds its own data structures:The OS adds its own data structures:– partitions the disk into groups of cylinderspartitions the disk into groups of cylinders

– logical formattinglogical formatting (make the file system) (make the file system) includes the directory/file structure,includes the directory/file structure,

and lists of free and allocated pagesand lists of free and allocated pages


4. Boot Blocks4. Boot Blocks

Most book blocks hold a simple bootstrap Most book blocks hold a simple bootstrap loader whose only job is to load and execute loader whose only job is to load and execute a fuller bootstrap program from diska fuller bootstrap program from disk– the program is located in a fixed the program is located in a fixed

place on disk (place on disk (a boot partitiona boot partition)) book block

FAT

root directory

data blocks(subdirectories)

sector 0sector 1

MS-DOS


Bad BlocksBad Blocks A bad block is a defective sector.A bad block is a defective sector. The OS can mark bad blocks and then skip The OS can mark bad blocks and then skip

them, or maintain a list of bad blocks.them, or maintain a list of bad blocks.

Sector sparingSector sparing (forwarding) (forwarding)– substitute a (close) good block for the bad onesubstitute a (close) good block for the bad one

Sector slippingSector slipping– move related blocks to be contiguous if they move related blocks to be contiguous if they

contain a bad blockcontain a bad block


5. Swap space Management5. Swap space Management

Swap space algorithms use virtual Swap space algorithms use virtual memory (VM) memory (VM) – disk space treated as RAMdisk space treated as RAM– slowslow– management aim is to speed-up VM management aim is to speed-up VM

throughput throughput


Swap Space ImplementationSwap Space Implementation

As a file:As a file:– standard interfacestandard interface– easy to implementeasy to implement– inefficient use of physical memoryinefficient use of physical memory

As a special partition:As a special partition:– no need to impose directory/file structuresno need to impose directory/file structures– fasterfaster– hard to reconfigurehard to reconfigure


5.1. UNIX Swap Space5.1. UNIX Swap Space (4.3 BSD) (4.3 BSD)

Each process is assigned a swap space Each process is assigned a swap space which holds:which holds:– text segmentstext segments (for pages of the program) (for pages of the program)– data segmentsdata segments (for runtime data) (for runtime data)

The kernel uses two The kernel uses two swap mapsswap maps to track to track swap space usage in a process.swap space usage in a process.


Swap MapsSwap Maps Figs. 13.7 and 13.8, p.444

512K 512K 512K 32K

text segment swap map

data segment swap map

16K 32K 64K 128K


6. Disk Reliability6. Disk Reliability

Common approach is to use multiple disks.Common approach is to use multiple disks.

Disk strippingDisk stripping (interleaving) (interleaving)– store data store data spreadspread across several disks across several disks– reduces chance of complete failurereduces chance of complete failure– I/O data transfers can use parallelismI/O data transfers can use parallelism

continued


Mirroring (shadowing) Mirroring (shadowing) duplicatesduplicates data data across disksacross disks– costlycostly– parallelism can increase speedparallelism can increase speed

Block interleaved parityBlock interleaved parity– if data on one machine is corrupted, it can be if data on one machine is corrupted, it can be

recalculated from the remaining data and parity recalculated from the remaining data and parity information stored on the other machinesinformation stored on the other machines

RAIDRAID (Redundant Array of Independent Disks) (Redundant Array of Independent Disks)

oses: 13. secondary storage 1 operating systems v objectives –introduce issues such as disk...

Documents