09 - Multimedia File Systems - RWTH Aachen · Chapter 4.2: Multimedia File Systems Page 9 Traditional File Systems: Disk Scheduling In traditional file systems, efficient usage of

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Page 1Chapter 4.2: Multimedia File Systems

4.2: Multimedia File Systems• Traditional File Systems

• Multimedia File Systems• Disk Scheduling

Chapter 2: Representation of Multimedia Data

Chapter 3: Multimedia Systems – Communication Aspects and Services

Chapter 4: Multimedia Systems – Storage Aspects

• Optical Storage Media

• Multimedia File Systems




Why Multimedia File Systems?

Heterogeneous data types including digital audio, animations and video…• Consuming enormous storage space

• Media are delay-sensitive: when user plays out or records a time dependent multimedia data object, the system must consume or produce at a constant data rate

• High demands to access to hard disc

→ A new multimedia enabled file system is needed in two means:� Organization of media content on the server

� Scheduling strategies for access to the data




Disk Layout

The layout of a disk determines

• the way in which content is addressed• how much storage space on the media is actually addressable and usable• the density of stored content on the media

Tracks and sectors• A hard disk consists of one or more heads

• A hard disk is divided into tracksand further into sectors (512 Byte)

• The same track on all heads is called cylinder

• Storage of a file is done in terms of sectors • Unused space of a sector is wasted

• Easy mapping of file location informationto head movement and disc rotation

• Constant angular velocity (CAV), i.e. same access time to inner/outer tracks

• Access to a sector by a movable disk arm




Disk Layout

Zone Bit Recording

• In the “normal” way, a sector at an outer radius has the same (sector) data amount, but more raw capacity. In principle, by this space is lost.

• Current approach for solution is zone bit recording

• Different read/write speeds, depending on the radius, allowing uniform sector size

• Place more popular media (movies) on an outer track to reduce average seek time, less popular media on an inner track. This saves disk arm movements.

Now: how to place files on such a disc?




Use of Storage Medium

Important: reduce read and write times by• fewer seek operations• lower rotational delay or latency

• high actual data transfer rate (can not be improved by placement)

Method: store data in a specific pattern

• Divide file in blocks (can be bytes, or of larger size)• Store blocks in certain patterns

• Larger block size• Fewer seek operations

• Smaller number of requests• But higher loss of storage space due to internal fragmentation (last block used only

50% on its sector on the average)




Traditional File Systems - File Structure

Non-contiguous Placement

Contiguous Placement

1st file 2nd file 3rd file

1st file

2nd file

3rd file

How to place the records of a file?




Performance Consideration of File StructureContiguous Placement:• Disk access time for reading and writing is minimized

• Major disadvantage: file creation, deletion and size modification makes this sequential storing difficult

Non-Contiguous Placement (two main approaches):1. Linked Allocation:

• Using pointers for addressing the next block

• Fine for sequential access

• Random access is costly• Long seek operations during playback

2. Indexed Allocation: • Links are stored in an index-block

• Complex• Performance depends on the index

structure and size of the file

1 6 82 4 5 73

beginning pointer

(first block is 1)(next block is 2)

1 6 82 4 5 73

12386




Traditional File Systems: Disk Management

Disk access is slow and costly - major bottleneck

Techniques reducing overall disk access time:• Block caches

� Keep blocks in memory for future use� Reduces the number of disk access

• Reduce disk arm motion� Blocks to be accessed in sequence are

placed on the same cylinder

� Reduces the time for one disk access� Take rotation into account by placing

consecutive blocks in an interleaved manner

• Placement of mapping tables� Mapping tables are placed

in the middle of the disk

� Tables and the corresponding blocks are placed on the same cylinder

Interleaved Storage

Non-interleaved Storage

Heads may read in parallel




Traditional File Systems: Disk Scheduling

In traditional file systems, efficient usage of storage capacity is the main goal. The total time to service a request to a file in such a system consist of:

• Seek time, head positioning to appropriate track (diameter)

• Latency (rotation time), time to find the block in the track• Actual data transfer time

Technique to reduce delay:• Seek operation → Scheduling algorithms

• Latency → File allocation methods

Next, we will consider strategies for minimizing the seek time, i.e. for the positioning time of the head to the appropriate track. Tracks are numbered 0, ..., N - 1. Here, 0 is the innermost and N - 1 the outermost track.

Del

ay




Disk Scheduling: First-Come-First-Served (FCFS)

Serve requests in order of arrival133

0 198108513113 6963 130 173

i+2

i

i+1

Overall movement counted in number of tracks visited for FCFS (in an example scenario): 673

(51)108173

31130

13133

6369

ii+1i+2

Queue

orde

r of

su

cces

sive

req

uest

s

+ Easy to implement+ Fair algorithm- Not optimal → High average seek time




Disk Scheduling: Shortest-Seek-Time First (SSFT)

SSFT = Serve “nearest“ request

Queue

(51)108173

31130

13133

6369Optimal overall

movement: 198

1330 108513113 6963 130 173

+ Substantial improvement over FCFS- Still not optimal

- Starvation (of some tracks if there is always a track with shorter seek time available)

SSTF movement: 243




Disk Scheduling: SCAN

• SCAN = serve requests in one direction; then reverse the movement

• Move from one end to the other, serving each request on the way

Queue

(51)108173

31130

13133

6369

0 19810851311369

63 130133

173

Overall movementSCAN: 224

Head Start




Disk Scheduling: Circular-SCAN (C-SCAN)

+ Fair service - More uniform waiting time - Performance not as good as SCAN + Middle tracks don’t get a better service than edge tracks (such as with SCAN or with SSTF)

C-SCAN is similar to SCAN but returns immediately to the beginning if the end is reached;one idle head movement from one edge to the other between two consecutive scans

Queue 0 198108513113 6963 130133 173

(51)108173

31130

13133

6369

Overall MovementC-SCAN: 376




Multimedia File System

Requirements of continuous data:

• File size:

� Highly structured data units (e.g. video and associated audio)→ New organization policies of the data on disk

� Efficient usage of limited storage is necessary

• Multiple data stream:� For example: retrieval of a movie requires the processing and synchronization

of audio and video data

• File access:� High, continuous throughput

� Short maximum (not average) response times

• Real-time characteristic:

� Stream play-out in constant, gap-free rate → additional buffers




“Multimedia” Disk Scheduling Algorithms

G

6 Milliseconds for 3 blocks of data

S + G

→ play back rate: 0.5 ms/block

e.g. G = 3rD = 2rC = 0,5results in…(S+G)/2 ≤ G/0.5S+G ≤ 12S ≤ 9

D C

S G Gr r+ ≤

playback duration

i.e. time to skip over a gap and to read the next media block is smaller than or equal to the duration of the playback

Restrictions of data placementHow to place media blocks?

Parameters• The size of a media block (granularity parameter G)

• # blocks: separation between successive blocks (scattering parameter S)

Continuity requirementrD data transfer rate from diskrC playback rate




Disk Scheduling Algorithms

To fulfill the requirements of multimedia data, scheduling has another focus than in traditional file systems:

• Goals in Traditional File Systems:� Reduce cost of seek time (effective utilization of disk arm)

� Achieve fair throughput� Provide fair disk access

� Achieve short average response times

• Goals in Multimedia File Systems are different:

� Meet deadlines of all time-critical tasks� Keep the necessary buffer space requirements low

� Find balance between time constraints and efficiency




Disk Scheduling: Earliest Deadline First (EDF)

• Poor throughput due to excessive seek time. Only deadlines are taken into account, but not track number.

• Very similar to FCFS: inefficient. Does not reflect the geographical position of tracks.

t 3 24

3 30

2 16

3 50

2 42

1 45

1 12

2 40

1 22

1 12

2 40

1 22

22 12 45 40 42 16

deadline track no.

1 45

1 12

2 40

2 16

3 50

2 42

3 30

2 16

3 50

3 50

2 42

2 40

2 42

1 45

2 40

In EDF the block with the nearestdeadline is read first.

Equal deadlines → FCFS




Disk Scheduling: SCAN-EDF

SCAN-EDF is a combination of:• Deadline scheduling (as in EDF earlier deadlines are served first)

• Scanning (tasks with same deadline are served according to the actual scan direction)

Problem: SCAN (i.e. use of scanning directions for tie break among equal deadlines) does not make much sense if too many different deadlines exist

Thus:• It has to be enforced that many requests have the same deadline

• In order to do so, all requests are grouped in a few groups which can be scanned together

• We require that deadlines Di are multiples of a common period p → Di ∈ {1, 2, 3, ...}• Then deadlines with the same period can be grouped and served together by SCAN





Implementation of SCAN-EDF by Perturbation of deadlines (in order to apply EDF)

• Let Di the deadline of task i and Ni be the track number (0 ≤ Ni < Nmax , e.g. Nmax = 100)• Assume that Di ∈• Modify Di towards Di’ (Di’ = perturbed deadline)

Di’ = Di + f(Ni)

• f(Ni) converts the track number of i into a small perturbation of the deadline such that for equal deadlines the scanning is automatically applied

If we choose (for example)

• Thus if the deadline for a task on track 42 is equal to 3 then the perturbed deadline is

• This deadline is given to the task at arrival time

ii

max

i

Nf (N )

N

0 f (N ) 1

=

⇒ ≤ <

421003 3,42+ =

�





• Among the same deadline SCAN is applied• Request with the earliest deadline is served• Sensible only for a large number of requests

• Optimization only applies for requests with the same deadline before the comma

• Increase this probability by grouping the requests

t2.16 16

3.50 50

2.42 42

1.45 45

1.12 12

2.40 40

1.22 22

12

22

45

40

16

1.12 12

2.40 40

1.22 22

1.45 45

2.40 40

1.22 22

2.42 42

2.40 40

1.45 45

3.50 50

2.42 42

2.40 40

2.16 16

3.50 50

2.42 42

deadline 1, i.e. ∈ [1:2]

deadline 2, i.e. ∈ [2:3]

Perturbed Deadline Track number




Disk Scheduling: EDF, SCAN-EDF

0 10 20 30 40 50

Dea

dlin

es

EDFSCAN-EDF




Disk Scheduling

A small variation of “deadline perturbation“:

• The actual deadline given to the task is refined by:� Taking into account the actual movement of the head at arrival time (i.e. upwards

from 0 to Nmax - 1 or downwards from Nmax - 1 to 0)

� Considering the actual position N of the head• The perturbed deadline for a task which resides on track Ni is given by: Di’ = Di + f(Ni)

where: i

imax

max ii

maxi

ii

max

ii

max

N NN N

N

N NN N

Nf(N )

NN N

N

N NN N

N

−⎧ ≥⎪⎪⎪ − <⎪⎪= ⎨⎪ >⎪⎪

−⎪ ≤⎪⎩

if and "head moves upwards"

if and "head moves upwards"

if and "head moves downwards"

if and "head moves downwards"

• This allows to serve new requests as soon as possible




Group Sweeping Scheduling (GSS)

• Requests are served in cycles in a round-robin manner• In one cycle requests are divided into groups. A group is served according to SCAN• Service in a group may be in ascending or in descending order depending on the

other groups• Thus a smoothing buffer may be needed (to assure continuity)

1.2 12

1.4 45

1.1 22

3.4 24

3.3 30

2.0 16

3.3 50

2.2 42

1.2 45

1.4 12

2.4 40

1.1 22

Cycle

Group 1 SCAN12, 22, 45

(ascending order)[in next cycle: descending order]

42, 40, 16(descending order)

2.0 16

2.2 42

2.4 40

Group 2 SCAN

3.4 24

3.3 30

3.3 50

Group 3 SCAN

Deadline 1.1

Deadline 2.0

24, 30, 50(ascending order)

t

Deadline 3.3




Group Sweeping Scheduling (GSS)

• A particular stream can be

� the first one in its group in a given cycle, but� the last one in its group in the next cycle

• This happens if the scan order is reversed, i.e. if we have an odd number of groups• Thus we need a smoothing buffer in order to achieve continuity of play-out

• GSS is a trade-off between optimization of buffer space and arm movements




Group Sweeping Scheduling (GSS) - Mixed Strategy

• The „mixed strategy“ is a compromise between

� Shortest seek (“greedy“)� Balanced strategy

• Data retrieved from disk are placed into buffers. Different queues are used for different data streams.

• “Shortest seek” serves the stream whose data block is nearest

• “Balanced” serves the stream which has the lowest utilization of buffers (since this stream risks to run out of data)




Group Sweeping Scheduling (GSS) - Mixed Strategy

• Filling status of buffers indicate when to switch from SSTF to “Balanced“ and vice versa

• “Urgency“ criterion:

• Fullness i = small → Urgency = high

→ Balanced strategy should be used

( i i= ∑

all streams )

1Urgency

Fullness




Conclusion

Multimedia Systems…• … is not only about the media format (MPEG, PCM, …)

• … also needs considerations how to store and access the media• … can be distributed: how to transmit the media over a network

• (… needs new user interfaces and programming concepts)

Multimedia in the future

• Distributed applications are becoming more important• Need of portability and system independence

• Better support for user interactivity

Variety of new (still undiscovered) application domains?

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09 - Multimedia File Systems - RWTH Aachen · Chapter 4.2: Multimedia File Systems Page 9 Traditional File Systems: Disk Scheduling In traditional file systems, efficient usage of