1. FILE MODELS CONTENT  INTRODUCTION  FILE MODELS  UNSTRURED AND STRUCTURED FILES  MUTABLE AND IMMUTABLE FILES

2. Introduction:  A file is a “Named object”.  That comes into existence by explicit creation.  The two main purposem of files  Permanent storage of information  Sharing of information

3. FILE MODELS  Different file system use the diffent conceptual models.  The two most commonly used for a file modeling are,  Structure  Mobility  File model based on these criterias.

4. Unstructured and structured files  It is the simplest model.  A file is an unstructured sequence of data.  There is no “Substructure”known to the file server.  The contents of each file of the file system appers to the file server as an “Uninterpreted sequence of data”.  The os is not interest in the information stored in the files.  Eg,UNIX and DOS use this file model.

5.  Another file model is rarely used the “Stucture file model”.  In this model,a file server is an “Ordered sequence of records”.  Records of different file of the same file system and can be different size.  Many type of files exist on file system each having different properties.  A record is the smallest unit of data can be accessed,a file system read or write operations are carried out on a ”Set of records”.

6.  Structured files are two types  Indexed records.  Nonindexed records.  A file record is accessed by specifying its positionWithin the file.  A file system that allow a indexed records.  A file is maintained as a B –tree or suitable data  Structure.  A hash table is used to locate records are quickly.

7.  Most modern systems use the “Unsturucred file model”.  This is mainly used for “Sharing of file” by different applicatin.  A file has a no structured in the unstructured model,different applications can interprete a file in different ways.  A file also normally have “Attributes”.  A file’s attributes are information describing that file.

8.  Each attribute has a name and value.  The types of attributes can be associated with a file are fixed by the file system.  File attributes are normally maintained by the “Directory service”.  They are stored with the corresponding file.  A many directory system allow files to be referenced by more than one name.

9. MUTABLE FILES: *****************  Most existing os use the mutable file model.  In this model,a update performed on a file overwrites.  Its old contents produce to new contents.  A file is represented as a single stored sequence that is already by each update operation.

10. I IMMUTABLE FILES *******************  Some more file system,such as (CFS)  [Gifford et al.1988]use the immutable model.  CFS-CEDAR FILE SYSTEM.  A file can’t be modified.  Once the file is created,except to be deleted.  To implement file updates,and each file is represented by a immutable versions.

11.  Giffort et al.[1988] emphasized that sharing only immutable files makes it easy to support “Consistent sharing”.  It is much easier to support file caching and replication in a distributed system.  Immutable file model eliminates all problems associated with multible versions of a file consistent.  The immutable file model suffers from two potential problem, * Incresed use of disk space. * Incresed a disk allocation activity.

12.  Prevent the disk space from filling instantaneously.  CFS[Giffort et al.1988] uses a “Keep” parameter as the no of most current versions of a file retained.  For eg,”demo.abc!6” is a name for version6.  The base name of “demo.abc”.  If the value of the keep parameter of a file is 1,the creation of new version of the file cause the file  “Demo.abc!6 “to be deleted.  And its diskspace to be reused for the new file named “demo.abc!7”.

13.  When the keep parameter value is greater than 1,for a file that is,when multible versions of a file exist.  Users may use a particular version of the file by specifying its full name.  When the version number is not specified,by default,CFS uses the lowest version number for  Some operations such as “Delete”.  And highest versin number for other operation such as “Open”.