Naming and Directories Andy Wang Operating Systems COP 4610 / CGS 5675.

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



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

/



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



The system needs to perform the following disk I/Os3. Read the file header for pets

/

pets

pets



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



The system needs to perform the following disk I/Os5. Read the file header for cat.jpg

/

pets

pets

cat

cat



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

Naming and Directories Andy Wang Operating Systems COP 4610 / CGS 5675.

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