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Naming and Directories

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1 Naming and Directories
Sarah Diesburg Operating Systems COP 4610

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 binder

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


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