CPSC 410/ 611: Week 9 Disk St ruct ure Disk Scheduling RAI D - - PDF document

CPSC 410 / 611 : Operating Systems 1

CPSC 410/ 611: “Week 9”

• Disk St ruct ure
• Disk Scheduling
• RAI D
• Disk Block Management
• Reading: Silberschat z Chapt er 12

CPSC 410/ 611: “Week 9”

• Disk St ruct ure
• Disk Scheduling
• RAI D
• Disk Block Management
• Reading: Silberschat z Chapt er 12

CPSC 410 / 611 : Operating Systems 2

Disk St ruct ure

cylinder

track sector

Disk speed:

• seek time : head moves to correct

track

• rotational delay : wait until sector is

under head

• transfer time : transfer data between

disk and memory

Disk Perf ormance

• Seek Time : Ts

n = number of t racks t raversed m = “t rack t raversal t ime” s = st art up t ime

• Rot at ional Delay (Lat ency Time): TR

r = # revolut ions per t ime unit

• Transf er Time: TT

b = # byt es t o be t ransf erred N = number of byt es on t rack

• Disk Access Time:

s n m TS + ´ = r TR 2 1 = rN b TT =

T R S

T T T T + + =

CPSC 410 / 611 : Operating Systems 3

CPSC 410/ 611: “Week 9”

• Disk St ruct ure
• Disk Scheduling
• RAI D
• Disk Block Management
• Reading: Silberschat z Chapt er 12

Disk Scheduling

• Quest ion: Does it pay of f t o t hink about scheduling policy in disk queue?
• Evaluat ion: Compare t ime f or service f or given request sequence,

dist inguish only by cylinder. application application device driver file system disk queue

CPSC 410 / 611 : Operating Systems 4

24 49 74 99 124 149 174 199

FCFS Scheduling

• Advant ages:

– simple – f air

• Disadvant ages

– poor average service t ime

• Example:

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

total head movement: 640 tracks 24 49 74 99 124 149 174 199

Short est -Seek-Time-First (SSTF)

• Always service closest request .
• P

roblem:

– St arvat ion

total head movement: 236 tracks

CPSC 410 / 611 : Operating Systems 5

SCAN (Elevat or Algorit hm)

• Cont inuously scan disk f rom one end t o t he ot her.
• When scanning, f ew request s af t er us, since j ust past t hrough.
• Problem: When we change direct ion at end, request s t here are very new.

24 49 74 99 124 149 174 total head movement: 236

C-SCAN (circular SCAN)

• Reduce variance in service t ime by always st art ing at t he

beginning of t he disk.

24 49 74 99 124 149 174

CPSC 410 / 611 : Operating Systems 6

LOOK, C-LOOK

24 49 74 99 124 149 174 total head movement: 322 24 49 74 99 124 149 174 total head movement: 299

CPSC 410/ 611: “Week 9”

• Disk St ruct ure
• Disk Scheduling
• RAI D
• Disk Block Management
• Reading: Silberschat z Chapt er 12

CPSC 410 / 611 : Operating Systems 7

RAI D

• Secondary st orage devices are slow!
• I mprove t heir perf ormance by using mult iple devices in parallel:

arrays of disks.

• RAI D

– Redundant Arrays of I ndependent Disks – Redundant Arrays of I nexpensive Disks (Berkeley)

• Common charact erist ics:

– Array of physical disks t hat are visible as single device t o OS. – Dat a is dist ribut ed across physical drives of array – Redundant disk capacit y is used f or error det ect ion/ correct ion

RAI D (cont )

• Replace single large-capacit y disk wit h array of smaller -capacit y

disks:

• Benef it s:

– I mproved I / O perf ormance – I ncrement al upgrade

• Problems:

– Reliabilit y: more devices increase t he probabilit y of f ailure. – Solut ion: redundancy

∫

∞ −

= = = > = ) ( ] [ ) ( . . ] [ ) ( dt t R t E MTTF e t R g e t t P t R

F t F λ

CPSC 410 / 611 : Operating Systems 8

RAI D Level 0

• No Redundancy
• St riping:

block blocks

Small strips Large strips

RAI D Level 1

• Adds redundancy t o Level 0: Mirroring

mirrors

• Problem:

– cost (100% redundancy)

• Perf ormance

– READs : good (replicat ion!) – WRI TEs: not (much) bet t er t han RAI D 0.

CPSC 410 / 611 : Operating Systems 9

RAI D Level 2

• Head and spindles synchronized
• Small st rips
• Error correct ion code calculat ed
• ver bis of dat a disks.

(Hamming Code)

• Appropriat e f or syst ems wit h

many f ailures.

• Typically not implement ed.

block

RAI D Level 3

• Heads and spindles

synchronized.

• Small st rips.
• Simple parit y bit s inst ead of

ECC. block parity Disk 1 fails:

1 2 3 4

) ( . . S S S S S S P g e ⊕ ⊕ ⊕ = =

2 3 4 1

S S S S S ⊕ ⊕ ⊕ =

CPSC 410 / 611 : Operating Systems 10

RAI D Level 4

• No synchronizat ion across

disks.

• Lar ge st r ips.
• Each st rip on parit y disk

cont ains parit y inf ormat ion f or all corresponding st rips. block

) ( ' 1 ) ( 1 ) ( 4 ) ( 1 ) ( 1 ) ( ) ( ' 1 ) ( 2 ) ( 3 ) ( ) ( ' 1 ) ( 2 ) ( 3 ) ( ' 4 ) ( ) ( 1 ) ( 2 ) ( 3 ) ( 4 i X i X i X i X i X i X i X i X i X i X i X i X i X i X i X i X i X i X i X ⊕ ⊕ = ⊕ ⊕ ⊕ ⊕ ⊕ = ⊕ ⊕ ⊕ = ⊕ ⊕ ⊕ =

CPSC 410/ 611: “Week 9”

• Disk St ruct ure
• Disk Scheduling
• RAI D
• Disk Block Management
• Reading: Silberschat z Chapt er 12

CPSC 410 / 611 : Operating Systems 11

Disk Format t ing

• Bare disk:
• Physical f ormat t ing:
• “cut ” int o sect ors
• ident if y sect ors
• add space f or error det ect ion/ correct ion
• Logical f ormat t ing:
• add blank direct ory, FAT, f ree space list , ...

ecc 1 ecc 2 ecc 3 ecc 4 ecc 5 ecc X 1 2 3 4 5 X FAT DIR DIR F/L ...

Framing

• Charact er count
• St art ing and ending f lags, wit h bit

st uf f ing

• Physical layer coding violat ions
• St art ing and ending chars, wit h

charact er st uf f ing

5 1 2 3 4 8 1 2 3 4 5 6 7 1 2

charact er count

DLE STX a b DLE DLE c DLE ETX

st uf f ed DLE

f raming pat t ern: 01111110

011011111011111011111010010

st uf f ed bit s

binary Manchest er

lack of t ransit ion

CPSC 410 / 611 : Operating Systems 12

Bad Block Management

• One or more blocks become unreadable/ unwrit eable: bad blocks
• Of f -line management of bad blocks:

– Run bad-block det ect ion program and put bad blocks on bad-block list . (Eit her remove t hem f rom f ree list or mark ent ry in FAT.) – May have t o run f ile recovery ut ilit y.

• On-line management :

– Have t he device driver map t he bad block ont o a good block – Block X goes bad. Whenever OS request s block X, t he disk t ransparent ly accesses a replacement block Y. – Problem: scheduling!