Copyright © by Curt Hill

Types of data External data Characters, numbers and other people readable stuff Eg: Reports to print Source programs Documents Binary data Things not in a people readable format such as machine language Copyright © by Curt Hill

Classification An operating system may type files according to one of several schemes Programs Commands Data UNIX believes in two types of files: Regular files Directories DOS follows UNIX Other categorizations are also possible Copyright © by Curt Hill

Things that may determine file types
Organization Operations allowable Whim of authors Copyright © by Curt Hill

Operations allowable What operations are acceptable for a particular file Deleting a file is different than a directory What naming conventions are acceptable? DOS requires .exe or .com for programs UNIX is not so particular Copyright © by Curt Hill

System Functions and Components
The following services need to be provided: Creation and deletion Manipulation Copying, renaming I/O operations Open/Close Read/Write There may be multiple kinds EOF test Secondary storage management Security Backup and recovery Copyright © by Curt Hill

Layers These services are provided in layers in a way similar to the OS layers Bottom level is the actual device Storage I/O control contains device drivers, interupt handlers etc Part of kernel At this level you see physical manipulation of the device What you do not see is files as logical entities Copyright © by Curt Hill

Upper Layers File control contains basic file services Open, close, read, write Allocation of storage space This is the bridge between the physical view of files and the logical view of files Copyright © by Curt Hill

The file hierarchy The smallest unit of storage is one character (at the file level) A field is composed of one or more characters Contains one unit of information Eg, name, price, text A record is one or more fields that apply their information to the same person or thing being described Copyright © by Curt Hill

Hierarchy Continued A file is one or more records In most cases each record is of same type and size though it may be the case that there are differing record types in the file A database is one or more files The files may be logical files in one physical file or different physical files A file system will generally have one or more files and/or databases Copyright © by Curt Hill

Hierarchy yet again The OS might or might not have some of the upper concepts UNIX and DOS do not distinguish a database from a file Only the program treats a file (or set of files) as a database VMS does know what a data base is Will not let an application treat it as a simple file Copyright © by Curt Hill

Storage allocation On a disk device we have a large number of storage allocation units These may be: Sectors Clusters (groups of sectors) Tracks Cylinders Since a disk is a direct access device we can access any of these without bothering any neighboring units Copyright © by Curt Hill

DOS FAT We need a record of these units such as DOS’s File Allocation Table This tells us: Which are free are which are used What belongs to whom This generally numbers all the units from zero to some maximum number Generally units with close numbers are physically close in some way Copyright © by Curt Hill

Close Sectors Close translates to rapid access Two sectors in the same track are close in that they only take rotation to access one when starting at the other Two tracks in the same cylinder are close in the sense that we only have to switch heads and not reposition Adjacent cylinders are close in that it takes a small amount of positioning to reach Copyright © by Curt Hill

Contiguous In contiguous the units of the file are together All the file units except the first and last must be surrounded by file units Usually in this type of file allocation you need to announce estimated size before you allocate, so that it can allocate an adequate block This is a problem with a poor estimate Copyright © by Curt Hill

Fragmentation Fragmentation can cause problems since we need to have one place to put a whole file Compaction is usually the answer It needs to be able to recognize absolute disk addresses in the file, if they are allowed Consider how a Indexed Sequential file links to the next item Such an item may be marked as unmovable or there must be something to aid relocation This problem is bypassed if the disk address is relative to the beginning of the file Copyright © by Curt Hill

How to Link scatter storage
Each unit points to the next unit The directory entry can have a list of units The disk sector map (FAT in DOS) may also have it This makes each entry a pointer to another entry as opposed to just a bitmap Then there is a free list as well Copyright © by Curt Hill

Blocking and Buffering
We have already discussed this Mechanism for speeding I/O The OS should manage so it is transparent to the user Copyright © by Curt Hill

File organization methods
We have also discussed this, some time ago Sequential Entry sequenced Direct Position is the key Indexed sequential Key can be anything Copyright © by Curt Hill

Directory contents Protection or access priviledges This might include the number of processes that have access Miscellaneous Dates and times Number of accesses since some time ago Copyright © by Curt Hill

Two Level directories A central directory with pointers toward a series of other directories The files of that second entry constitute the second level I think CMS and NOS used this style Mac OS recently did something like this Copyright © by Curt Hill

Tree structured directories
UNIX introduced tree shaped directories, though it may have borrowed them from Multics For each disk device there is a root which is the main directory A directory may contain files or other directories Branching can occur as often as desired Copyright © by Curt Hill

Tree Structured Again DOS 2 and up has adopted this approach UNIX adds one more wrinkle: A user becomes a directory Any files or directories rooted in that directory then belong to that user Access controls of a directory are generally inherited from the directory it is in UNIX puts public important things in directories in the root Copyright © by Curt Hill

Tree Structured Paths It also needs the idea of a path Command search path: A list of directories that will be searched for commands and/or files File specification Relative and absolute paths Copyright © by Curt Hill

Graph directories Acyclic Very similar to tree shaped but allows sharing in a different way Two tree branches can arrive at the same node (file) thus allowing sharing of that file The Acyclic name comes from the idea that you can never traverse a node twice in any path The links of UNIX and NTFS allow this form General does allow cycles This looks more like a network database than most file systems Copyright © by Curt Hill

Directory Implementation
How do we represent a directory? Array (linear list) of items This is the DOS approach Limits the number of files in a directory The list is not sorted Sorted lists Same as above but sorted Linked list Tree (embedded in a table) Copyright © by Curt Hill

File and Directory manipulation
What should be allowed? Read and Write of the directory Edit Delete Security and File integrity issues Copyright © by Curt Hill

Ownership and access controls
How is the access to files regulated On single user systems there is seldom any such controls On multi-user systems it becomes an OS task to control how others may access a user’s file UNIX has a good example system so we will examine Copyright © by Curt Hill

UNIX access controls For each file or directory there are a group of 9 bits that determine access The three categories of access are: Read Write Execute Furthermore those who would access the file are partitioned into three groups which can have separate R/W/E access Owner Group World Copyright © by Curt Hill

Notes The owner by default has full access, but can restrict his own access, as desired Group A group of users that can access group accessible information The system administrator determines the group that you are in, usually by your own request Any user can only be in one group World Any user of the system Copyright © by Curt Hill

System administration
Number of topics: File migration File backup Transaction logging File recovery Copyright © by Curt Hill

File migration How do we keep track of different versions? VMS has a version number applied to each item DOS program traditionally use .BAK or some other extension to represent the last one There was a master number in OS/MVS files of certain types What do we do with the old ones? Eventually they need to be moved off of disk Copyright © by Curt Hill

System Administration Again
File backup Full backup involves the entire disk Incremental involves just those things with a newer date Transaction logging Journal transactions File recovery Remove error sectors from a file Copyright © by Curt Hill

Copyright © by Curt Hill

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