File models and file accessing models
- 1. Prepared By: Mehta Ishani 1300407010030 File models and file accessing models
- 2. Contents 2 File models Unstructured and structured files Mutable and immutable files File accessing models Accessing remote files Unit of data transfer
- 3. Unstructured files 3 Simplest model File – unstructured sequence of data No substructure Contents – un interpreted sequence of bytes Unix, 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.
- 4. Structured files 4 Rarely used File – ordered sequence of records Files – different types, different size and different properties Record – smallest unit of data that can be accessed Two categories Files with non indexed records Files with indexed records
- 5. Structured files 5 Files with non indexed records File records is accessed by specifying it’s position within file For ex. Fifth record from beginning, second record from end Files with indexed records Records have one or more key fields that can be addressed by specifying values File is maintained as B-tree or other suitable data structure or hash table to locate records quickly
- 6. File attributes 6 Information describing that files Has name and value Contains 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
- 7. Mutable files 7 Used by most existing os Update performed on files overwrites on old contents to produce new contents File is represented as a single stored sequence and that is altered by each update operation
- 8. Immutable files 8 Cedar files system File 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
- 9. Immutable files 9 Suffering from two issues Increased use of disk space Increased disk allocation activity CFS uses keep parameter as the no. of most current version of file to be retained.
- 10. Accessing remote files 10 One of the following model is used when request to access remote file Remote service model Data catching model
- 11. Remote service model 11 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
- 12. Data catching model 12 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
- 13. Unit of data transfer 13 Refers to fraction of file data that is transferred to and from client as a result of single read write operation Four data transfer models File level transfer model Block level transfer model Byte level transfer model Record level transfer model
- 14. File level transfer model 14 When the operation required file data, the whole file is moved Advantages are Efficient because network protocol overhead is required only once Better 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 space Ex are amoeba, CFS, Andrew file system
- 15. Block level transfer model 15 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 performance Ex are Sun microsystem’s NFS, Apollo domain file system
- 16. Byte level transfer model 16 File data transfer take place in units of bytes Provides maximum flexibility Difficulty in cache management
- 17. Record level transfer model 17 Suitable with structured files File data transfer take place in unit of records Ex. RSS(research storage system)
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