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Naming and Directories
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Recall from the last time…
File system components Disk management organizes disk blocks into
files. Many disk blocks management schemes A file header associates the file with its data
blocks Naming provides file names and directories to
users. Protection Reliability
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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
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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
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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
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Naming
Naming: allows users to issue file names instead of i-node numbers A mapping from names (paths) to I-nodes Similar to the DNS in the Internet.
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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 Is this right?
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Name Space
Flat name space Hierarchical naming Relational name space Contextual naming Content-based naming
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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
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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
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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
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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
?
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An Example of Path Resolution
To access the data content of /pets/cat.jpg
The system needs to perform the following disk I/Os
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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
/
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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
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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
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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
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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
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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
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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
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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
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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?
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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
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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
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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
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Example: The “Internet File System”
Can be viewed as a worldwide file system What is the naming scheme for the Internet
file system?
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The “Internet File System”
Contains shades of various naming schemes Flat name space:
Each website 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
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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