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25-Distributed_Systems_III_2spp

25-Distributed_Systems_III_2spp - CSE 421/521 Operating...

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CSE 421/521 - Operating Systems Fall 2011 Tevfik Ko ş ar University at Buffalo December 1 st , 2011 Lecture - XXV Distributed Systems - III What does Distributed File System Provide? 2 Provide access to and manipulation of data stored at remote servers using file system interfaces What are the file system interfaces? Open a file, check status on a file, close a file; Read data from a file; Write data to a file; Lock a file or part of a file; List files in a directory, delete a directory; Delete a file, rename a file, add a symlink to a file; i.e. POSIX interface
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Why is DFS Useful? 3 Data sharing of multiple users User mobility Data location transparency Data location independence Replications and increased availability Not all DFS are the same: Local-area vs Wide area DFS Fully Distributed FS vs DFS requiring central coordinator File System vs Block-Level Interface 4 Data are organized in files, which in turn are organized in directories Compare these with disk-level access or “block” access interface: [Read/Write, LUN, block#] Key differences: Implementation of the directory/file structure and semantics – Synchronization
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Buzz Words: NAS vs SAN 5 NAS SAN Access Methods File access Disk block access Access Medium Ethernet Fiber Channel and Ethernet Transport Protocol Layer over TCP/IP SCSI/FC and SCSI/IP Efficiency Less More Sharing and Access Control Good Poor Integrity demands Strong Very strong Clients Workstations Database servers Naming of Distributed Files Naming – mapping between logical and physical objects. A transparent DFS hides the location where in the network the file is stored. Location transparency file name does not reveal the file’s physical storage location. File name denotes a specific, hidden, set of physical disk blocks. Convenient way to share data. Could expose correspondence between component units and machines. Location independence – file name does not need to be changed when the file’s physical storage location changes. Better file abstraction. Promotes sharing the storage space itself. Separates the naming hierarchy from the storage-devices hierarchy.
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DFS - Three Naming Schemes 1. Mount remote directories to local directories, giving the appearance of a coherent local directory tree Mounted remote directories can be accessed transparently.
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