Post on 18-Jan-2016
Naming and Directories
Andy Wang
Operating Systems
COP 4610 / CGS 5675
Recall from the last time…
A file header associates the file with its data blocks
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
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
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
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…
How do you name these photos?
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
Name Space
Flat name space Hierarchical naming Relational name space Contextual naming Content-based naming
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
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
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
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
?
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os1. Read in the file header for the root directory ‘/’
Stored at a fixed location on disk
/
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os2. Read the first data block for the root directory
Lookup the directory entry for pets
/
pets
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os3. Read the file header for pets
/
pets
pets
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os4. Read the first data block for the pet directory
Lookup the directory entry for cat.jpg
/
pets
pets
cat
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os5. Read the file header for cat.jpg
/
pets
pets
cat
cat
An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os6. Read the data block for cat.jpg
/
pets
pets
cat
cat
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
Relational Name Space
Hierarchical naming model is largely a tree Relational naming model 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
Pros/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?
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
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
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
Example: The “Internet File System”
Can be viewed as a worldwide file system What is the naming scheme for the Internet
file system?
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
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