3dfsimpl - Distributed File System Implementation...

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Distributed File System Implementation Satyanarayanan (1981) made a study of file usage patterns. Some of the measurement are static -- meaning that they represent a snapshot of the system at a certain instant. the distribution of file sizes the distribution of file types the amount of storage occupied by files of various types and sizes. Other measurements are dynamic -- made by modifying the file system to record all operations to a log for subsequent analysis. the relative frequency of various operations the number of files open at any moment the amount of sharing that take place By combining the static and dynamic measurements, we can get a better picture of how the file system is used.
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File usage What a typical user population is, is always a problem. Satyanarayanan’s measurements were made at a university. How about industrial research labs, office automation projects, banking systems, no one knows. Another problem inherent in making measurements is watching out for artifacts of the system being measured. A simple example, when looking at the distribution of file names in an MS- DOS system, one could quickly conclude that file names are never more than 8 + 3 characters. It would be a mistake to draw the conclusion that 8 characters are enough. Finally Satyanarayanan’s measurements were made on more-or-less traditional UNIX systems. Whether or not they are the same when projected to a distributed system is a big unknown.
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File Usage Observed file system properties Most files are small (less than 10 K) -- transfer the whole file instead of blocks Reading is much more common than writing -- caching improves the performance Reads and writes are sequential; random access is rare -- local caching Most files have short lifetime -- create files on client side File sharing us unusual -- local caching with session semantics The average process uses only a few files Distinct file classes with different properties exist System binaries need to be widespread but hardy ever change, so they can be widely replicated. Scratch files are short, unshared, and disappear quickly, so they should be kept locally. Electronic mailboxes are frequently updated but rarely shared, so replication is not likely to gain anything. Ordinary data files may be shared, so they may need still other handling.
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System Structure Are clients and servers different? In some systems, there is no distinction between clients and servers. All machines run the same basic software, any machine can feel free to offer file service to the public. (windows 95 or NT) Offering file service is just a matter of exporting the names of selected directories so that other machines can access them. In other system, the file server and directory server are just user programs, so
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This note was uploaded on 09/17/2009 for the course IT it771 taught by Professor Jenisha during the Fall '09 term at University of Advancing Technology.

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3dfsimpl - Distributed File System Implementation...

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