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Naming and Directories

Naming and Directories. Sarah Diesburg Operating Systems CS 3430. 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

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Naming and Directories

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  1. Naming and Directories Sarah Diesburg Operating Systems CS 3430

  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. More recently • 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 to 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. pets cute ? ? 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

  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, too many cat pics, etc • - 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 • Google is starting to do this • Good idea? Bad idea?

  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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