1. Chapter 10: File-System Interface
What is a file-system?
An abstract representation of physical devices capable of storing and retrieving binary information (files)
consists of partitions (volumes (NT), minidisks, etc), directories, and files
“virtual” file-systems may be smaller or greater in size then the physical devices used to store the information
What is a file?
A file is an abstract data type, defined by the operations that can be performed on them
A named collection of related information recorded on a secondary storage (non-volatile) device
Uniform logical view of information storage
A logical storage unit (mapped by the OS onto a physical device)
Who decides how the meaning is encoded (structure)?
Operating system
Program
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2. CEG 433/633 - Operating Systems I
File Attributes
A file is a sequence of bits, bytes, lines or records whose format encodes meaning as defined by the file’s creator or user:
“Freeform” structure: sequence of words, bytes
Simple regular structure: fixed or variable length records
Complex structures: formatted document, relocatable load file
File types:
Data (numeric, character, binary)
Executable (loader can bring into memory and execute)
Directory (keeps information about other files)
Attributes:
Name – only information kept in human-readable form.
Type – needed for systems that support different types.
Location – pointer to file location on device.
Size – current file size.
Protection – controls who can do reading, writing, executing.
Time, date, and user identification – data for protection, security, and usage monitoring.
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3. File Types – name, extension
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4. CEG 433/633 - Operating Systems I
Access Methods
Sequential Access
read next
write next
reset
no read after last write
(rewrite)
Direct Access
read n
write n
position to n
rewrite n
n = relative block number
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5. CEG 433/633 - Operating Systems I
File Operations
Files are an abstract data type, defined by the operations that can be performed on the. The OS must provide system calls for the following types of operations:
create: find space, write to file, update directory
write: name and position, update size, update file pointer
read: name and position, update file pointer
seek: reposition within file, update file pointer
delete: deallocate space, erase directory entry
truncate
open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory.
close (Fi) – move the content of entry Fi in memory to directory structure on disk.
Files must be explicitly opened via open() to create a entry in the open-file table. The index to the entry in the open-file table is the “file handle”.
# of processes w/ current-file position pointer (offset) for each
Information regarding physical location of file on disk
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6. CEG 433/633 - Operating Systems I
Directory Structure
A collection of nodes containing information about all files.
Operations:
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
Directory
F 1
F 2
F 3
F 4
F n
Files
Both the directory structure and the files reside on disk.
Backups of these two structures are kept on tapes.
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7. Single-Level Directory
A single directory for all files (all users).
Naming problem
Each file must have a unique name -
it may be difficult to keep track of “used” names
What if two users want to use the same filename?
What if two users have the _same_ file under two different names?
Grouping problem
files are not grouped by logical properties (C programs, games, etc.)
Efficiency – how can you locate a file quickly (if you don’t know the name…)
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8. CEG 433/633 - Operating Systems I
Two-Level Directory
Separate directory for each user.
Path name
Can have the same file name for different user
Efficient searching
No grouping capability
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9. Tree-Structured Directories
Efficient searching
Grouping Capability
Current directory (working directory)
cd /spell/mail/prog
type list
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10. Tree-Structured Directories (Cont.)
Absolute or relative path name
Creating a new file is done in current directory.
Delete a file
rm <file-name>
Creating a new subdirectory is done in current directory.
mkdir <dir-name>
Example: if in current directory /spell/mail
mkdir count
mail
prog
copy
prt
exp
count
Deleting “mail”  deleting the entire subtree rooted by “mail”.
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11. Acyclic-Graph Directories
Have shared subdirectories and files.
Different names (aliasing)
Solutions:
Backpointers, so we can delete all pointers. Variable size records a problem.
Backpointers using a daisy chain organization.
Entry-hold-count solution.
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12. General Graph Directory
How do we guarantee no cycles?
Allow only links to file not subdirectories.
Garbage collection.
Every time a new link is added use a cycle detection algorithm to determine whether it is OK.
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13. CEG 433/633 - Operating Systems I
Protection
File owner/creator should be able to control:
what can be done
by whom
Types of access
Read
Write
Execute
Append
Delete
List
Access Lists
Access Control Lists (ACLs)
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14. Access Lists and Groups in UNIX
Mode of access: read, write, execute
Three classes of users
RWX
a) owner access 7  RWX
b) groups access 6  1 1 0
c) public access 1  0 0 1
Ask manager to create a group (unique name), say G, and add some users to the group.
For a particular file (say game) or subdirectory, define an appropriate access.
owner
group
public
chmod
761
game
Attach a group to a file
chgrp G game
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