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

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1 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 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 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. Unix/Linux with NFS; Windows with mapped drives
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This note was uploaded on 02/27/2012 for the course CSE 421/521 taught by Professor Binar during the Fall '11 term at SUNY Buffalo.

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

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