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Page 1: CHAPTER 14: MASS-STORAGE STRUCTURE Disk Structure Disk Structure Disk Scheduling Disk Scheduling Disk Management Disk Management Swap-Space Management

CHAPTER 14: MASS-STORAGE STRUCTURE Disk StructureDisk Structure Disk SchedulingDisk Scheduling Disk ManagementDisk Management Swap-Space ManagementSwap-Space Management RAID StructureRAID Structure Disk AttachmentDisk Attachment Stable-Storage ImplementationStable-Storage Implementation Tertiary Storage DevicesTertiary Storage Devices

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DISK STRUCTURE Disk drives are addressed as large 1-dimensional Disk drives are addressed as large 1-dimensional

arrays of arrays of logical blockslogical blocks, where the logical block is the , where the logical block is the smallest unit of transfer. smallest unit of transfer.

The 1-dimensional array of logical blocks is mapped The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially.into the sectors of the disk sequentially. Sector 0 is the first sector of the first track on the Sector 0 is the first sector of the first track on the

outermost cylinder.outermost cylinder. Mapping proceeds in order Mapping proceeds in order

through that track, through that track, then the rest of the tracks in that cylinder, and then the rest of the tracks in that cylinder, and then through the rest of the cylinders from then through the rest of the cylinders from

outermost to innermost.outermost to innermost.

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DISK SCHEDULING The operating system is responsible for using The operating system is responsible for using

hardware efficiently — for the disk drives, this means hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth.having a fast access time and disk bandwidth.

Disk bandwidth is the total number of bytes Disk bandwidth is the total number of bytes transferred, divided by the total time between the first transferred, divided by the total time between the first request.request.

Access time has two major componentsAccess time has two major components Seek timeSeek time is the time for the disk to move the heads is the time for the disk to move the heads

to the cylinder containing the desired sector.to the cylinder containing the desired sector. Latency timeLatency time is the additional time waiting for the is the additional time waiting for the

disk to rotate the desired sector to the disk head.disk to rotate the desired sector to the disk head. Minimize seek time Minimize seek time Minimize seek distance. Minimize seek distance. Minimize seek time Minimize seek time Maximize disk bandwidth. Maximize disk bandwidth.

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Disk Scheduling

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

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

98, 183, 37, 122, 14, 124, 65, 6798, 183, 37, 122, 14, 124, 65, 67

Head pointer 53Head pointer 53

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Disk Scheduling: FCFS

The total head movement of 640 cylinders.

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Disk Scheduling: SSTF Selects the request with the minimum seek time from Selects the request with the minimum seek time from

the current head position.the current head position. SSTF scheduling is a form of SJF scheduling; may SSTF scheduling is a form of SJF scheduling; may

cause starvation of some requests.cause starvation of some requests. 236 cylinders.236 cylinders.

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Disk Scheduling: SCAN Sometimes called the Sometimes called the elevator algorithmelevator algorithm.. The head continuously scans back and forth across the The head continuously scans back and forth across the

disk. disk. 208 cylinders.208 cylinders.

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Disk Scheduling: C-SCAN C-SCAN (Circular SCAN)C-SCAN (Circular SCAN) Provides a more uniform wait time than SCAN.Provides a more uniform wait time than SCAN. Treats the cylinders as a circular list that wraps around from the Treats the cylinders as a circular list that wraps around from the

last cylinder to the first one.last cylinder to the first one.

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Disk Scheduling: Look / C-Look Similar to SCAN/C-SCANSimilar to SCAN/C-SCAN Arm only goes as far as the last request in each Arm only goes as far as the last request in each

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

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Disk Scheduling: Selecting a Algorithm

SSTF is common and has a natural appealSSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that SCAN and C-SCAN perform better for systems that

place a heavy load on the disk.place a heavy load on the disk. Performance depends on the number and types of Performance depends on the number and types of

requests.requests. Requests for disk service can be influenced by the Requests for disk service can be influenced by the

file-allocation method.file-allocation method. The disk-scheduling algorithm should be written as a The disk-scheduling algorithm should be written as a

separate module of the operating system, allowing it separate module of the operating system, allowing it to be replaced with a different algorithm if necessary.to be replaced with a different algorithm if necessary.

Either SSTF or LOOK is a reasonable choice for the Either SSTF or LOOK is a reasonable choice for the default algorithm.default algorithm.

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DISK MANAGEMENT: Disk formatting Disk formattingDisk formatting

Low-Level FormattingLow-Level FormattingDividing a disk into sectors that the disk Dividing a disk into sectors that the disk

controller can read and write.controller can read and write.Every sector consists of a header, a data area Every sector consists of a header, a data area

(usually 512 in size), and a trailer.(usually 512 in size), and a trailer.The data area size can be chosen. The data area size can be chosen. The header and trailer contain information used The header and trailer contain information used

by disk controller, such as a sector number and by disk controller, such as a sector number and an error-correcting code (ECC).an error-correcting code (ECC).

Low-level formatting is usually done by vendors. Low-level formatting is usually done by vendors. High-Level FormattingHigh-Level Formatting

Partition the hard diskPartition the hard diskBuild the metadata structures for a FS.Build the metadata structures for a FS.

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Disk Management: Boot block Booting processBooting process

CPU self-testingCPU self-testing Run the bootstrap at the ROM (BIOS for PC)Run the bootstrap at the ROM (BIOS for PC) Load the first block from the bootable partitionLoad the first block from the bootable partition

Boot BlockBoot Block Default by MS-DOS/MS WindowsDefault by MS-DOS/MS Windows Lilo (FS unware)Lilo (FS unware) Grub (FS ware)Grub (FS ware) Other boot: Linux in LinuxOther boot: Linux in Linux

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Disk Management: Boot block (FAT)

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Disk Management: Bad blocks For floppy disk, the head moves on the disk surface.For floppy disk, the head moves on the disk surface. For hard disk, the head flies over the disk surface. For hard disk, the head flies over the disk surface. Bad blocksBad blocks

For IDE, bad blocks are handled manually. For IDE, bad blocks are handled manually. For SCSI, bad blocks are handled smartly.For SCSI, bad blocks are handled smartly.

Sector sparingSector sparing Sector slipping.Sector slipping.

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SWAP-SPACE MANAGEMENT: Usage Swap space is used to extend the physical memory.Swap space is used to extend the physical memory. Swap space can be used to holdSwap space can be used to hold

Entire processEntire process Part of process such as pagesPart of process such as pages

Swap space size Swap space size Overestimating is saferOverestimating is safer Underestimating is dangerous. Underestimating is dangerous.

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Swap-Space Management: Location Can be a normal file: Can be a normal file:

Win3.1Win3.1 Windows 2K/XPWindows 2K/XP

Can be a partitionCan be a partition SolarisSolaris LinuxLinux

Can be bothCan be both SolarisSolaris LinuxLinux

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Swap-Space Management: An example SunOSSunOS

4.3BSD allocates swap space when process starts; 4.3BSD allocates swap space when process starts; holds holds text segmenttext segment (the program) and (the program) and data segment.data segment.

Solaris 2 Kernel uses Solaris 2 Kernel uses swap mapsswap maps to track swap- to track swap-space use.space use.

Solaris 2 allocates swap space only when a page is Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the forced out of physical memory, not when the virtual memory page is first created.virtual memory page is first created.

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RAID How to improve the performanceHow to improve the performance How to improve the reliabilityHow to improve the reliability

Single disks Single disks multiple disks multiple disks RAID RAID RAID (Redundant Array of Inexpensive Disks)RAID (Redundant Array of Inexpensive Disks) RAID (Redundant Array of Independent Disks)RAID (Redundant Array of Independent Disks)

OutlineOutline ReliabilityReliability PerformancePerformance LevelsLevels [Which level is suitable][Which level is suitable] [Extensions][Extensions]

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RAID: Reliability Without redundancyWithout redundancy

Suppose that the mean time to failure of a single Suppose that the mean time to failure of a single disk is 100,000 hours, disk is 100,000 hours, the mean time to failure the mean time to failure of some disk in an array of 100 disks will be of some disk in an array of 100 disks will be 100,000/100 = 1000 hours = 41.66 days. 100,000/100 = 1000 hours = 41.66 days.

With redundancyWith redundancy MirroringMirroring

pp is failure probability for one disk is failure probability for one diskp*pp*p is the failure probability for two disks at the is the failure probability for two disks at the

same time. same time.

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RAID: Performance Without parallelismWithout parallelism

One block after one blockOne block after one block With parallelismWith parallelism

Double disksDouble disks Double the speedDouble the speed

Multiple disksMultiple disksMultiple the speed.Multiple the speed.

Data striping: Data striping: bit-level, bit-level, [byte-level], [byte-level], block-levelblock-level

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RAID: Levels

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RAID: Levels

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DISK ATTACHMENT How to attach a diskHow to attach a disk

Host-attached storageHost-attached storage Network-attached storageNetwork-attached storage Storage-area networkStorage-area network

Host-attached storageHost-attached storage IDE(ATA): Two drives per I/O busIDE(ATA): Two drives per I/O bus SCSI (Small Computer System Interface): SCSI (Small Computer System Interface):

One bus: SCSI initiator One bus: SCSI initiator 15 SCSI targets 15 SCSI targets 15 15*8 logical units (usually disks) per I/O bus*8 logical units (usually disks) per I/O bus

FC (Fibre Channel)FC (Fibre Channel)One variant address 126 devicesOne variant address 126 devicesAnother variant address 2^24 devices. Another variant address 2^24 devices.

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Disk Attachment

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Disk Attachment

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STABLE-STORAGE IMPLEMENTATION Write-ahead log scheme requires stable storage.Write-ahead log scheme requires stable storage.

To implement stable storage:To implement stable storage: Replicate information on more than one Replicate information on more than one

nonvolatile storage media with independent nonvolatile storage media with independent failure modes.failure modes.

Update information in a controlled manner to Update information in a controlled manner to ensure that we can recover the stable data ensure that we can recover the stable data after any failure during data transfer or after any failure during data transfer or recovery.recovery.

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TERTIARY STORAGE DEVICES Low cost is the defining characteristic of Low cost is the defining characteristic of

tertiary storage.tertiary storage.

Generally, tertiary storage is built using Generally, tertiary storage is built using removable mediaremovable media

Common examples of removable media are Common examples of removable media are floppy disks and CD-ROMs; other types are floppy disks and CD-ROMs; other types are available.available.

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Tertiary-Storage Devices: Removable Disks Floppy disk — thin flexible disk coated with Floppy disk — thin flexible disk coated with

magnetic material, enclosed in a protective magnetic material, enclosed in a protective plastic case.plastic case.

Most floppies hold about 1 MB; similar Most floppies hold about 1 MB; similar technology is used for removable disks that technology is used for removable disks that hold more than 1 GB.hold more than 1 GB.

Removable magnetic disks can be nearly as Removable magnetic disks can be nearly as fast as hard disks, but they are at a greater fast as hard disks, but they are at a greater risk of damage from exposure.risk of damage from exposure.

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Tertiary-Storage Devices: Removable Disks A magneto-optic disk records data on a rigid platter A magneto-optic disk records data on a rigid platter

coated with magnetic material.coated with magnetic material. Laser heat is used to amplify a large, weak Laser heat is used to amplify a large, weak

magnetic field to record a bit.magnetic field to record a bit. Laser light is also used to read data (Kerr effect).Laser light is also used to read data (Kerr effect). The magneto-optic head flies much farther from the The magneto-optic head flies much farther from the

disk surface than a magnetic disk head, and the disk surface than a magnetic disk head, and the magnetic material is covered with a protective layer magnetic material is covered with a protective layer of plastic or glass; resistant to head crashes.of plastic or glass; resistant to head crashes.

Optical disks do not use magnetism; they employ Optical disks do not use magnetism; they employ special materials that are altered by laser light.special materials that are altered by laser light.

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Tertiary-Storage Devices: Removal Disks: WORM The data on read-write disks can be modified over and The data on read-write disks can be modified over and

over.over. WORM (“Write Once, Read Many Times”) disks can WORM (“Write Once, Read Many Times”) disks can

be written only once.be written only once. Thin aluminum film sandwiched between two glass or Thin aluminum film sandwiched between two glass or

plastic platters.plastic platters. To write a bit, the drive uses a laser light to burn a To write a bit, the drive uses a laser light to burn a

small hole through the aluminum; information can be small hole through the aluminum; information can be destroyed by not altered.destroyed by not altered.

Very durable and reliable.Very durable and reliable. Read OnlyRead Only disks, such ad CD-ROM and DVD, come disks, such ad CD-ROM and DVD, come

from the factory with the data pre-recorded.from the factory with the data pre-recorded.

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Tertiary-Storage Devices: Tapes Compared to a disk, a tape is less expensive and holds Compared to a disk, a tape is less expensive and holds

more data, but random access is much slower.more data, but random access is much slower. Tape is an economical medium for purposes that do Tape is an economical medium for purposes that do

not require fast random access, e.g., backup copies of not require fast random access, e.g., backup copies of disk data, holding huge volumes of data.disk data, holding huge volumes of data.

Large tape installations typically use robotic tape Large tape installations typically use robotic tape changers that move tapes between tape drives and changers that move tapes between tape drives and storage slots in a tape library.storage slots in a tape library. stacker – library that holds a few tapesstacker – library that holds a few tapes silo – library that holds thousands of tapes silo – library that holds thousands of tapes

A disk-resident file can be A disk-resident file can be archivedarchived to tape for low to tape for low cost storage; the computer can cost storage; the computer can stagestage it back into disk it back into disk storage for active use. storage for active use.

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Tertiary-Storage Devices: Future Technology

Holographic storageHolographic storage Lots of dataLots of data Fast (in one flash of laser light)Fast (in one flash of laser light)

MEMS (Micro Electronic Mechanical Systems)MEMS (Micro Electronic Mechanical Systems) To produce electronic chips to manufacture small To produce electronic chips to manufacture small

data storage machines. data storage machines. For example, to make an array of 10,000 tiny disk For example, to make an array of 10,000 tiny disk

heads.heads.

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Tertiary-Storage Devices: Application Interface Removable disk (raw I/O and FS):Removable disk (raw I/O and FS):

Most OSs handle removable disks almost exactly like fixed diMost OSs handle removable disks almost exactly like fixed disks — a new cartridge is formatted and an empty file system is sks — a new cartridge is formatted and an empty file system is generated on the disk.generated on the disk.

Tapes (raw I/O):Tapes (raw I/O): Tapes are presented as a raw storage medium, i.e., and applicatTapes are presented as a raw storage medium, i.e., and applicat

ion does not not open a file on the tape, it opens the whole tape ion does not not open a file on the tape, it opens the whole tape drive as a raw device.drive as a raw device.

Usually the tape drive is reserved for the exclusive use of that aUsually the tape drive is reserved for the exclusive use of that application.pplication.

Since the OS does not provide file system services, the applicatSince the OS does not provide file system services, the application must decide how to use the array of blocks.ion must decide how to use the array of blocks.

Since every application makes up its own rules for how to orgaSince every application makes up its own rules for how to organize a tape, a tape full of data can generally only be used by the nize a tape, a tape full of data can generally only be used by the program that created it.program that created it.

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Tertiary-Storage Devices: File Naming The issue of naming files on removable media iThe issue of naming files on removable media i

s especially difficult when we want to write dats especially difficult when we want to write data on a removable cartridge on one computer, aa on a removable cartridge on one computer, and then use the cartridge in another computer. nd then use the cartridge in another computer.

Contemporary OSs generally leave the name spContemporary OSs generally leave the name space problem unsolved for removable media, anace problem unsolved for removable media, and depend on applications and users to figure oud depend on applications and users to figure out how to access and interpret the data.t how to access and interpret the data.

Some kinds of removable media (e.g., CDs) are Some kinds of removable media (e.g., CDs) are so well standardized that all computers use theso well standardized that all computers use them the same way.m the same way.

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Tertiary-Storage Devices: Hierarchical Storage Management (HSM) A hierarchical storage system extends the storage A hierarchical storage system extends the storage

hierarchy beyond primary memory and secondary hierarchy beyond primary memory and secondary storage to incorporate tertiary storage — usually storage to incorporate tertiary storage — usually implemented as a jukebox of tapes or removable disks.implemented as a jukebox of tapes or removable disks.

Usually incorporate tertiary storage by extending the Usually incorporate tertiary storage by extending the file system.file system. Small and frequently used files remain on disk.Small and frequently used files remain on disk. Large, old, inactive files are archived to the jukebox.Large, old, inactive files are archived to the jukebox.

HSM is usually found in supercomputing centers and HSM is usually found in supercomputing centers and other large installations that have enormous volumes of other large installations that have enormous volumes of data.data.

CDROM TowersCDROM Towers

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Tertiary-Storage Devices: Speed Two aspects of speed in tertiary storage are Two aspects of speed in tertiary storage are

bandwidth and bandwidth and latency.latency.

Bandwidth is measured in bytes per second.Bandwidth is measured in bytes per second. Sustained bandwidth – average data rate during a Sustained bandwidth – average data rate during a

large transfer; # of bytes/transfer time.large transfer; # of bytes/transfer time.Data rate when the data stream is actually flowing.Data rate when the data stream is actually flowing.

Effective bandwidth – average over the entire I/O Effective bandwidth – average over the entire I/O time, including time, including seekseek or or locatelocate, and cartridge , and cartridge switching.switching.Drive’s overall data rate.Drive’s overall data rate.

The bandwidth of a drive is generally understood to The bandwidth of a drive is generally understood to mean the sustained bandwidth. (0.5MBS mean the sustained bandwidth. (0.5MBS 5MBS)5MBS)

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Tertiary-Storage Devices: Speed Access latency – amount of time needed to locate data.Access latency – amount of time needed to locate data.

Access time for a disk – move the arm to the selected cylindAccess time for a disk – move the arm to the selected cylinder and wait for the rotational latency; < 35 milliseconds.er and wait for the rotational latency; < 35 milliseconds.

Access on tape requires winding the tape reels until the seleAccess on tape requires winding the tape reels until the selected block reaches the tape head; tens or hundreds of secondcted block reaches the tape head; tens or hundreds of seconds.s.

Generally say that random access within a tape cartridge is Generally say that random access within a tape cartridge is about a thousand times slower than random access on disk.about a thousand times slower than random access on disk.

The low cost of tertiary storage is a result of having many cheaThe low cost of tertiary storage is a result of having many cheap cartridges share a few expensive drives.p cartridges share a few expensive drives.

A removable library is best devoted to the storage of infrequentA removable library is best devoted to the storage of infrequently used data, because the library can only satisfy a relatively sly used data, because the library can only satisfy a relatively small number of I/O requests per hour.mall number of I/O requests per hour.

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Tertiary-Storage Devices: Reliability A fixed disk drive is likely to be more reliable than a A fixed disk drive is likely to be more reliable than a

removable disk or tape drive.removable disk or tape drive.

An optical cartridge is likely to be more reliable than a An optical cartridge is likely to be more reliable than a magnetic disk or tape.magnetic disk or tape.

A head crash in a fixed hard disk generally destroys A head crash in a fixed hard disk generally destroys the data, whereas the failure of a tape drive or optical the data, whereas the failure of a tape drive or optical disk drive often leaves the data cartridge unharmed.disk drive often leaves the data cartridge unharmed.

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Tertiary-Storage Devices: Cost Main memory is much more expensive than disk Main memory is much more expensive than disk

storagestorage

The cost per megabyte of hard disk storage is The cost per megabyte of hard disk storage is competitive with magnetic tape if only one tape is competitive with magnetic tape if only one tape is used per drive.used per drive.

The cheapest tape drives and the cheapest disk drives The cheapest tape drives and the cheapest disk drives have had about the same storage capacity over the have had about the same storage capacity over the years.years.

Tertiary storage gives a cost savings only when the Tertiary storage gives a cost savings only when the number of cartridges is considerably larger than the number of cartridges is considerably larger than the number of drives.number of drives.

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Tertiary-Storage Devices: Price per Megabyte of DRAM, From 1981 to 2000

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Tertiary-Storage Devices: Price per Megabyte of Disk, From 1981 to 2000

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Tertiary-Storage Devices: Price per Megabyte of Tape, From 1981 to 2000


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