Naming and Directories - PowerPoint PPT Presentation

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Naming and Directories

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Uses multiple levels of directories. Most popular name space organization ... Store all files under a directory within a cylinder to avoid disk seeks ... – PowerPoint PPT presentation

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Title: Naming and Directories


1
Naming and Directories
2
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

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
  • 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.

7
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?

8
Name Space
  • Flat name space
  • Hierarchical naming
  • Relational name space
  • Contextual naming
  • Content-based naming

9
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

10
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

11
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
12
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

13
An Example of Path Resolution
  • To access the data content of /pets/cat.jpg
  • The system needs to perform the following disk
    I/Os

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
  • 1. Read in the file header for the root directory
    /
  • Stored at a fixed location on disk

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
  • 2. Read the first data block for the root
    directory
  • Lookup the directory entry for pets

pets
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
  • 3. Read the file header for pets

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
  • 4. Read the first data block for the pet
    directory
  • Lookup the directory entry for cat.jpg

pets
pets
cat
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
  • 5. Read the file header for cat.jpg

pets
cat
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
  • 6. Read the data block for cat.jpg

pets
cat
pets
cat
20
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

21
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

22
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?

23
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

24
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

25
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

26
Example The Internet File System
  • Can be viewed as a worldwide file system
  • What is the naming scheme for the Internet file
    system?

27
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

28
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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