Operating SystemsECE344

Ashvin GoelECE

University of Toronto

Disks and RAID

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Outline

Diskso Disk scheduling algorithmso Redundancy in storage systems, RAID

File Systemso Overview of file systemo File system designo Consistency and crash recoveryo Sharing fileso Unix file system

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Disk Geometry

Disks have multiple platterso Each platter has an heado The different heads are attached to a single armo The different heads can access data in parallel

Each platter has multiple concentric trackso A cylinder consists of the same track across different platters

Each track has multiple sectorso A sector typically has preamble, data and ECC

Track

Sector Cylinder

Platters

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Disk Access Delays

Time to access a disk sector is determined by 3 delayso Seek time

Time to move head to correct track

o Rotational delay Time for disk to rotate to

correct sectoro Transfer time

Time to read/write the bits of sector

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Disk Performance Trends

Capacityo 100% per year (2X every year)

Transfer rate (BW)o 40% per year (2X every two years)o Typically, 50-100 MB/so Sector transfer time is 100-200 microseconds

Seek time and Rotation timeo 8% per year (1/2 every 10 years)o Seek and rotation time are typically 4-8 ms todayo Disk scheduling aims to minimize these times, especially seek

time

Disk Performance

Fastest: No head movement, no rotational latencyo => Sequential access

Faster: no head movement, rotational latencyo => Access to blocks on the same cylinder is faster

Slow: head movement, rotational latencyo => Access to blocks on different cylinderso => The further the cylinders the worse it is

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Addressing Disks

Older disks required OS to specify all parameters for transferring datao E.g., cylinder #, track #, sector #, transfer size

Modern disks are more complicatedo Not all sectors are the same size, sectors are remapped, etc.

Current disks provide a higher-level interfaceo Disk exports its data as a logical array of blockso Disk maps logical blocks to its surfaceo OS code is simpler but disk parameters are hidden

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Layers of Abstraction

Program <Filename, Offset> File system <Partition, Block#> Device driver <Disk#, Sector#> Disk Controller <Cylinder, Track, Sector>

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Disk Errors

Lots of errors possibleo E.g., latent sector errors, mis-directed writes, etc.o Transient vs. hard errorso Some errors can be masked by ECC

Bad sectorso Allocate spare sectors per tracko Block can be mapped to spare in various ways

In the factory, by the device controller, by OSo OS can hide bad sectors by allocating them to a special

hidden fileo Physical backup programs have to careful

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Disk Scheduling Algorithms

Aim is to improve disk performance Two methods

o Reduce seeks and rotationo Read several blocks of data

Algorithmso First-come, first served (FCFS)

Simple, fair, slowo Shortest seek time first (SSF)o SCAN (Elevator)

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Shortest Seek First (SSF)

Shortest seek first minimizes arm motion Unlike FCFS, starvation is possible

Initialposition

Pendingrequests

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SCAN (Elevator)

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SCAN (Elevator)

Use a bit to track outward or inward arm direction Service the next pending request in same direction When there are no more requests in the current

direction, reverse direction Increases seek compared to SSF but ensures no

starvation

A variant is called C-SCANo SCAN, but request go in one direction (typewriter)o What are its benefits/drawbacks vs. SCAN?

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Modern Disk Scheduling

Disks know their layout better than the OS May ignore or undo disk scheduling in OS!

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Redundancy in Storage Systems

Idea: Use many disks in parallelo Increases storage bandwidth, improves reliability

Redundant Array of Inexpensive Disks (RAID)o A storage system, not a file system

Files are striped across diskso Stripes on different disks can be read/written in parallelo Bandwidth increases with more diskso Better throughput for large requests

Choosing stripe size is importanto Normally between 1KB to 1MBo Increase stripe size, as average file size increases

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RAID Levels 0, 1

RAID level 0: disk stripingo Distributes data across several disks for speedo No redundancy

RAID level 1: mirroringo Backup solutiono Write both, read eithero 50% utilization

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RAID Levels 0, 1

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RAID Levels 4, 5

RAID level 4: block striping, dedicated parity disko Calculate XOR value of stripes across diskso Store XOR stripe value on separate disko Uses one extra disk for parity

Utilization: (N-1)/N, N is the number of disks

RAID level 5: striping with distributed parityo Similar to 4 but parity information is distributed across all

diskso Avoids bottleneck for parity disko If a single disk dies, it has to be replaced

Information for that disk is recreated from the other disks

RAID level 6:o Can recovery from the loss of two diskso Allows (single) failure during disk recovery

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RAID Levels 4, 5