A Virtual Shared Disk (VSD) facility is a layer of software that allows a processing node to access disk devices physically attached to a remote node as if those devices were attached locally. Thus, nodes are presented with the illusion of shared disks. Furthermore, one can have access to a disk from more nodes than the disk has tails, which is important for scalability in SP systems. Finally, if a disk has more than one tail, and the primary tail/node fails, requests are routed automatically to a secondary tail. The routing is totally transparent to applications, which do not see a failure; they may experience just a small I/O delay.

For multimedia support, VSD has been enhanced to support real-time access; deadline scheduling is performed dynamically at the VSD server nodes. Real-time VSD (RTVSD) is currently in use in a scalable parallel (SP) video server trial in Japan. Using the real-time aspects of the RTVSD allows the filesystems to make guarantees of service for real-time applications such as interactive audio and video, which make it integral to a scalable multimedia server. Recoverable VSD allows provides transparent access to data in the presence of VSD server failures.

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Real-Time Virtual Shared Disk

Real-Time VSD provides the underlying data engine used by the Tiger Shark File System in the Clustered Web Server.

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Rajat Mukherjee <rajatm@watson.ibm.com>

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