1

Operating Systems

Part VI: Mass-Storage Structure

2

Disk Structure

PlatterCylinderTrackSector

3

Data Transfer

Seek time: time to get to the track/cylinder containing the sector

Rotational latency: time to get to the desired sector

Disk bandwidth: number of bytes transferred per unit time (a.k.a. transfer rate)

4

Disk Scheduling

FCFS (First Come First Served)– Simplest to implement– Intrinsically fair– Does not provide the fastest service– Example

98,183,37,122,14,124,65,67Assume start at cylinder 53

5

Disk Scheduling

FCFS (First Come First Served)

6

Disk Scheduling

SSTF (Shortest Seek Time First)– Selects the request with the minimum seek

time from the current head position– Essentially a form of SJF– May cause starvation– Not optimal, but substantial improvement over

FCFS

7

Disk Scheduling

SSTF (Shortest Seek Time First)

8

Disk Scheduling

SCAN Scheduling– Sometimes called the elevator algorithm– Starts at one end of disk and moves toward

other end, servicing requests as it reaches each cylinder, until it gets to other end

– Head continuously scans back and forth– Disadvantage: When head reverses direction,

what happens to density of request in vicinity?

9

Disk Scheduling

SCAN Scheduling

10

Disk Scheduling

C-SCAN Scheduling– Variation of SCAN but returns to beginning of

disk without servicing requests on return trip– Treats cylinder as circular list that wraps

around from final cylinder to the first one

11

Disk Scheduling

C-SCAN Scheduling

12

Disk Scheduling

LOOK and C-LOOK Scheduling– Variations of SCAN and C-SCAN– Goes as far as final request in each direction

then reverses course without going all the way to the end

– Looks for request before continuing in a direction

13

Disk Scheduling

C-LOOK Scheduling

14

Selection of Disk Scheduling Algorithm

Depends heavily on number and type of request

Can be heavily influenced by file allocation method (contiguous vs. linked/indexed)

Location of directory (first/middle/last cylinder) -> better cached

SSTF or LOOK reasonable default

15

Disk Management

Formatting– Low-level formatting (physical formatting)

Fills disk with special data structure (header, data area, and trailer) for each sector (usually 512 bytes) -> usually done in factory

Implements error-correcting code (ECC) which is updated with value calculated from all bytes in sector

When sector is read, ECC is recalculated and compare with stored value -> mismatch = corruption

16

Disk Management

Formatting (continued)– Partitioning

Divides disk into one or more groups of cylindersTools: fdisk, Partition Magic, System Commander

– Logical formattingCreation of file system (FAT/inode, free-space

mapping, initial empty directory)Raw disk: large sequential array of disk blocks

without any file system structure (used by some applications) -> bypasses directory structure, file names, space allocations, and other file services

17

Disk Management

Boot Block– Boot short for bootstrap– Boot program stored in ROM

Fixed location and needs no initializationCannot be infected by virusProblem: changing boot code requires changing

ROM hardware chips

– Solution:ROM -> boot disk (w/ boot partition)-> O/S kernel

18

Disk Management

Bad Blocks– Unavoidable due to moving parts and proximity of

R/W head to platter surface– Data in bad blocks are usually lost– Handled manually in simple disks (IDE)

Format scans for bad blocks and locks them away Chkdsk and scandisk do the same but used in normal

operation

– SCSI disks “smarter” in handling bad blocks Uses sector sparing or forwarding Sector slipping sometimes used in place of sparing

19

Swap Space

Goal: provide best throughput for virtual memory system

Swap space use– Varies depending on O/S (e.g. entire process image,

pages pushed out of MM swapped, etc.)– Some O/Ss allow multiple swap spaces– Safer to overestimate than to underestimate size

Swap space location– Part of the file system (Windows)– Separate disk partition (normally raw device, e.g. Unix)

20

RAID Structure

Redundant Arrays of Inexpensive Disks Concept now extended to other devices such as

tapes Redundancy solves problem of reliability

– Duplicate each disk (mirroring/shadowing)

Performance improvement via parallelism– Mirroring doubles read access times– Data striping: splitting data across multiple disks (bit-

level, block-level, sector-level, etc.)

21

RAID Levels

Mirroring: high-reliability but expensive; striping: high-speed but does not improve reliability

RAID levels provide a scheme that combines mirroring and striping– Cost-performance trade-offs (hence, levels)– Example: RAID 0 (block striping but without

mirroring), RAID 1 (mirroring), RAID 0 + 1

22

Disk Attachment

Host-attached storage– May be IDE, SCSI, or RAID

device

Network-attached storage– Provides convenient way to

share pool of storage and data– Consumes network bandwidth– More efficient if Storage Area

Network (SAN) is implemented (using storage rather than networking protocols)

23

Tertiary Storage

Built with removable mediaLow-cost is defining characteristicRemovable disks

– Magnetic (e.g. floppy, zip drives, hot-swappable hard-disks)

– OpticalWORM (Write Once Read Many, e.g. CD-R)Phase-change (e.g. CD-RW)

– Magneto-optical

24

Tertiary Storage

Tapes– Off-line: sits on shelves– Near-line: uses robotic arms -> between off-

line and on-line (e.g. disks)Future technology

– Holographic storage: records holographic photographs on special media

– Micro-electronic mechanical systems (MEMS) -> produces small storage machines

25

Tertiary Storage Performance

SpeedReliabilityCost