The EXT3NS is a scalable file system designed to handle video streaming workload in large-scale on-demand streaming services. It is based on a special H/W device, called Network-Storage card (NS card), which aims at accelerating streaming operation by shortening the data path from storage device to network interface. The design objective of EXT3NS is to minimize the delay and the delay variance of I/O request in the sequential workload on NS card. Metadata structure, file organization, metadata structure, unit of storage, etc. are elaborately tailored to achieve this objective. Further, EXT3NS provides the standard API's to read and write files in storage unit of NS card. The streaming server utilizes it to gain high disk I/O bandwidth, to avoid unnecessary memory copies on the data path from disk to network, and to alleviates CPU's burden by offloading parts of network protocol processing, The EXT3NS is a full functional file system based on the popular EXT3. The performance measurements on our prototype video server show obvious performance improvements. Specifically, we obtain better results from file system benchmark program, and obtain performance improvements in disk read and network transmission, which leads to overall streaming performance increase. Especially, the streaming server shows much less server's CPU utilization and less fluctuation of client bit rate, hence more reliable streaming service is possible.