Multimedia data includes
audio and video clips (i.e. MP3 and MPEG files)
live webcasts
Multimedia data may be delivered to desktop PC’s, handheld devices (PDAs, smart phones), …
Multimedia data is stored in the file system like otheordinary data.
However, multimedia data must be accessed with specific timing requirements.
For example, video must be displayed at 24-30 frames per second. Multimedia video data must be delivered at a rate which guarantees 24-30 frames/second.
Continuous-media data is data with specific rate requirements.
Streamings
Streamings delivering a multimedia file from a server to a client -typically the deliver occurs over a network connection.
There are two different types of streaming:
1. Progressive download-the client begins playback of the multimedia file as it is delivered. The file is ultimately stored on the client computer.
2. Real-time streaming-the multimedia file is delivered to -but not stored on -the client’s computer.
Streamings
There are two types of real-time streaming:
(1) Live streaming-used to deliver a live event while it is occurring.
(2) On-demand streaming-used to deliver media streams such as movies, archived lectures, etc. The events are not delivered in real-time..
Compression
Because of the size and rate requirements of multimedia systems,multimedia files are often compressed into a smaller form.
MPEG Compression:
(1) MPEG-1 -352 X 240 @ 30 frames/second
(2) MPEG-2 -Used for compressing DVD and high-definition television (HDTV)
(3) MPEG-4 -Used to transmit audio, video, and graphics. Can be delivered over very slow connections (56 Kbps)
Risorse di un file server
Scheduling
La tolleranza agli errori è meno stringente
Il non rispetto delle deadline in alcuni casi non è avvertibile
Usano soprattutto task periodici, più facili da gestire
Si può negoziare la banda disponibile o la qualità fornita (video scalabile);
Server Client Audio Video
Scheduling
Scheduling
Scheduling
Scheduling del disco
Soluzioni per ottimizzare l’accesso al disco
Data striping (es. RAID)
Algoritmi di scheduling Es.:
SCAN
SCAN-EDF
Riducono il costo di ricerca dei settori del disco
Permettono un throughput maggiore
Permettono il rispetto delle deadline
Scadenza (deadline) e cilindro (cylinder) richiesto per lo scheduling SCAN-EDF
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