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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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 2Chapter 4.2: 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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 3Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 4Chapter 4.2: Multimedia File Systems
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?
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 5Chapter 4.2: Multimedia File Systems
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)
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 6Chapter 4.2: Multimedia File Systems
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?
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 7Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 8Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 9Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 10Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 11Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 12Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 13Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 14Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 15Chapter 4.2: Multimedia File Systems
“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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 16Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 17Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 18Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 19Chapter 4.2: Multimedia File Systems
Disk Scheduling: SCAN-EDF
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+ =
�
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 20Chapter 4.2: Multimedia File Systems
Disk Scheduling: SCAN-EDF
• 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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 21Chapter 4.2: Multimedia File Systems
Disk Scheduling: EDF, SCAN-EDF
0 10 20 30 40 50
Dea
dlin
es
EDFSCAN-EDF
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 22Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 23Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 24Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 25Chapter 4.2: Multimedia File Systems
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)
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 26Chapter 4.2: Multimedia File Systems
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
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 27Chapter 4.2: Multimedia File Systems
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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