file models and file accessing models
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File models and file accessing modelsTRANSCRIPT
Prepared By:Mehta Ishani
1300407010030
File models and file accessing models
Contents
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File modelsUnstructured and structured filesMutable and immutable files
File accessing modelsAccessing remote filesUnit of data transfer
Unstructured files
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Simplest modelFile – unstructured sequence of dataNo substructureContents – un interpreted sequence of bytesUnix, MS-Dos Modern OS used this model because sharing of
files is easier in compared with structured file model
Since file has no structure then different applications can interpret the contents of files in many different ways.
Structured files
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Rarely used File – ordered sequence of recordsFiles – different types, different size and
different propertiesRecord – smallest unit of data that can be
accessedTwo categories
Files with non indexed recordsFiles with indexed records
Structured files
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Files with non indexed recordsFile records is accessed by specifying it’s
position within fileFor ex. Fifth record from beginning, second
record from end
Files with indexed recordsRecords have one or more key fields that can
be addressed by specifying valuesFile is maintained as B-tree or other suitable
data structure or hash table to locate records quickly
File attributes
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Information describing that filesHas name and valueContains information such has owner, size,
access permission, date of creation, date of last modification and date of last access
User can read the value of any attribute but can’t change modify it
Maintained and used by directory service because they are subject to different access control
Mutable files
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Used by most existing osUpdate performed on files overwrites on
old contents to produce new contentsFile is represented as a single stored
sequence and that is altered by each update operation
Immutable files
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Cedar files systemFile can not be modified once it has been
created except to be deleted file versioning approach is used, a new version
of file is created when change is made rather than updating same file
In practice storage space may be reduced by keeping only differences rather than created whole file again
Sharing is much easier because it supports caching and replication which eliminates the problem of keeping multiple consistent copies
Immutable files
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Suffering from two issues Increased use of disk spaceIncreased disk allocation activity
CFS uses keep parameter as the no. of most current version of file to be retained.
Accessing remote files
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One of the following model is used when request to access remote fileRemote service modelData catching model
Remote service model
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Processing of client request is performed at server’s node
Client request is delivered to server and server machine performs on it and returns replies to client
Request and replies transferred across network as message
File server interface and communication protocol must be designed carefully so as to minimize the overhead of generating the messages
Every remote file access results in traffic
Data catching model
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Reduced the amount of network traffic by taking advantage of locality feature
If requested data is not present locally then copied it from server’s node to client node and catching there
LRU is used to keep the cache size bounded
Cache Consistency problem
Unit of data transfer
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Refers to fraction of file data that is transferred to and from client as a result of single read write operation
Four data transfer modelsFile level transfer modelBlock level transfer modelByte level transfer modelRecord level transfer model
File level transfer model
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When the operation required file data, the whole file is moved
Advantages areEfficient because network protocol overhead is
required only onceBetter scalability because it requires fewer access
to file server and reduce server load and network traffic
Disk access routines on server can be better optimized
Offers degree of resiliency to server and network failure
Drawbacks is it requires sufficient storage spaceEx are amoeba, CFS, Andrew file system
Block level transfer model
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file data transfer take place in units of data blocks
A file block is contiguous portion of file and fixed in length
Advantage is does not required large storage space
Drawback is more network traffic and more network protocol overhead
Poor performanceEx are Sun microsystem’s NFS, Apollo
domain file system
Byte level transfer model
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File data transfer take place in units of bytes
Provides maximum flexibilityDifficulty in cache management
Record level transfer model
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Suitable with structured filesFile data transfer take place in unit of
recordsEx. RSS(research storage system)
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