1. Naming and Directories
Andy Wang
Operating Systems
COP 4610 / CGS 5675

2. Recall from the last time…
A file header associates the file with its data blocks

3. File Header Storage
Under UNIX, a file header is stored in a data structure called i-node
For early UNIX systems
I-nodes are stored in a special array
Fixed number of array entries
Maximum number of files fixed
Not stored near data blocks on disk
Reading a small file involves
One disk seek to get the i-node
Other disk seek(s) to get file blocks

4. Reasons for Separate Allocations
Reliability
Data corruptions are unlikely to affect i-nodes
Reduced fragmentation
File headers are smaller than a whole block
By packing them in an array, multiple headers can be fetched from disk
File headers are accessed more often
e.g., ls
Grouping file headers improves disk efficiency

5. For BSD 4.2… Portions of file header array stored on each cylinder
For small directories
All file headers and data stored in the same cylinder
Reduce seek time

6. Naming
Remember that odd moment when your computer asks you for name the first file?
Naming: allows users to issue file names instead of i-node numbers
- Users tend to come up with poor names
e.g., test
- Many file are difficult to name…

7. How do you name these photos?

8. Directories A table of file names and their i-node numbers
Under many file systems
Directories are implemented as normal files
Containing file names and i_node numbers
Only the OS is permitted to modify directories

9. Name Space Flat name space Hierarchical naming Relational name space
Contextual naming
Content-based naming

10. Flat Name Space All files are stored in a single directory
+ Easy to implement
- Not scalable for large directories
Name collisions: multiple files with the same names

11. Hierarchical Naming Uses multiple levels of directories
Most popular name space organization
+ Conceptual model maps well into the human model of organizing things
A file cabinet contains many files
+ Scalable
The probability of name collisions decreases
+ Spatial locality
Store all files under a directory within a cylinder to avoid disk seeks

12. More on Hierarchical Naming
Absolute path name: consisting the path from the root directory ‘/’ to the file
e.g., /pets/cat.jpg
root directory
sub directory
file name

13. Drawbacks of Hierarchical Naming
- Not all files can fit into the hierarchical model
- Accessing a file may involve many levels of directory lookups, or a path resolution before getting to the file content
pets ?
pests ?

14. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os

15. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
1. Read in the file header for the root directory ‘/’
Stored at a fixed location on disk /

16. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
2. Read the first data block for the root directory
Lookup the directory entry for pets /
pets

17. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
3. Read the file header for pets /
pets
pets

18. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
4. Read the first data block for the pet directory
Lookup the directory entry for cat.jpg /
pets
pets
cat

19. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
5. Read the file header for cat.jpg /
pets
cat
pets
cat

20. An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
6. Read the data block for cat.jpg /
pets
cat
pets
cat

21. Some Performance Optimizations…
Top-level directories are usually cached
A user inside a directory (e.g., /pets)
Can issue relative path names (e.g., cat.jpg) to refer files within the current directory

22. Relational Name Space Hierarchical naming model is largely a tree
One step beyond is the relational naming model, which allows the construction of general graphs
A file can belong to multiple folders
According to its attributes
Files can be accessed in a manner similar to relational databases
e.g., keywords: cats and blinds

23. Pros and Cons of Relational Name Space
+ More flexible than hierarchical naming
- May require a long list of attributes to name a single piece of data
e.g., this lecture
Keywords: operating systems, file systems, naming, PowerPoint XP
- Who will create those attributes?

24. Contextual Naming
Takes advantage of the observation that certain attributes can be added automatically
e.g., when you try to open a file by Word, a system will search only the file types supported by Word (.doc, .txt, .html)
+ Avoids a long list of attributes
- A user may not remember the file name

25. Content-Based Naming Searches a file by its content instead of names
File contents are extracted automatically
e.g., I want a photo of a cat taken five years ago
The system returns all files satisfying the criteria

26. Content-Based Naming
- Requires advanced information processing techniques
e.g., image recognition
Many existing systems use manual indexing
Automated content-based naming is still an active area of research

27. Example: The “Internet File System”
Can be viewed as a worldwide file system
What is the naming scheme for the Internet file system?

28. The “Internet File System”
Contains shades of various naming schemes
Flat name space:
Each URL provides a unique name
Hierarchical name space:
Within individual websites
Relational name space
Can search the Internet via search engines
Contextual name space:
Page ranked according to relevance
Content-based name space:
You can find your information without knowing the exact file names

29. Example: Plan 9
Modern UNIX has a deep-rooted influence from the Plan 9 OS
Developed by Bell lab
Major design philosophy: everything is a file
A single hierarchical name space for
Processes (e.g., /proc)
Files
IPC (e.g., pipe)
Devices (e.g., /dev/fd0)
Use open/close/read/write for everything
e.g., /dev/mem