multimedia files:
a digital movie may actually consist of many files:one video,multiple audio,and multiple texts(subtitles)，fast forward, fast backward.
It should be obvious that manipulating multimedia material in uncompressed form is completely out of the problem----it is too big.The only hope is that massive compression is possible.
在MPEG（IPB三种frame）压缩标准中，拖动滚动条定位想要播放的时刻是由I-frame支持，倒带由B-frame支持。
multimedia process scheduling:
general real-time scheduling:the number of users changes,frame sizes vary,resolutions are different,so different processes may have to run at different frequencies,with different amount of works,and with different deadline.The scheduling of competing processes,some or all of which have deadlines that must be met,is called real-time scheduling.
2 algorithms:RMS&EDF
multimedia FS:
not pull server model:open,read,read,read,....close.
but push server model:start,(pushing ....),stop
compression makes forward and backward complicated.
what can be done id to build extra files as fast forward and fast backward.
Near Video on Demand With VCR control
File Placement:
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