Notes/Operating Systems/File Systems Management.md

type, backlinks

type	backlinks
theoretical	Memory Management

A file system consists of two parts

• Collection of files
• A directory structure -> provides information about all files in the system

File

• Logical view -> the unit of storing data Files are mapped by the OS onto physical nonvolatile devices

Types:

• Data
  • Numeric
  • Character
  • Binary
• Program

Attributes:

• Name
• Identifier (unique number)
• Type¹
• Location -> pointer
• Size
• Protection (permissions)
• Datetime and user id

Logical Definition

• Named collection of related information
• Files may have free form (text files) or can be rigidly formatted²

Operations

• Create
• Write
• Read
• Seek (reposition within file)
• Delete
• Truncate - shorten or cut off by removing data from the end
• Open (load to memory)
• Close (unload)

Open files

Tracked by an open-file table, counted by file-open count.

In order to avoid race conditions, we need to lock the files somehow.

• Shared lock -> several processes can acquire concurrently, used for reads
• Exclusive lock -> writer lock
• Mandatory vs. advisory -> access is denied depending on locks held and requested vs. processes can find status of locks and decide what to do

Structure

Could be many:

• None
• Simple record
  • Lines
  • Fixed length
  • Variable length
• Complex
  • Formatted document
  • Relocatable load file ³

Directories

Collection of nodes containing information about all files. Also resides on disk.

Operations:

• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse file system

Single level directory

A single directory for all users.

Clearly, we need unique names, which can become a problem real fast. That shit is gonna grow super big.

Two-level directory

Users have different directories. In Linux -> /home/user is separate, allowing for the same file names. Linux, however, uses a multi-level:

Tree-Structured Directories

• Efficient searching
• Grouping
• Absolute v. relative path

Acyclic-Graph

Have shared subdirectories and files. Symlinks achieve this.

Symlinks

Hard vs Soft. Hard is a literal copy of the file but keep the same inode info, while soft is just a pointer.

Important

We only allow links to files to avoid cycles Every time a new link is added we also use a cycle detection algorithm to determine whether it is OK

Disk

Can be subdivided into partitions.

Disk/partition can be used raw (no file system) or can be formatted. The entity containing the file system is known as a volume.

[!NOTE]- Typical fs organization

Access lists and groups

Read, write and execute. Three classes of users on Linux

1. Owner -> 7 (Read Write Execute)
2. Group -> 6 (RW)
3. Public -> 1 (X)

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1. Extension (.pdf, .txt) as opposed to format, which specifies the grammar of the file ↩︎

2. Columnar, fixed-format ASCII Files have fixed field lengths, as opposed to delimited, i.e. fields can be as large as we want them to ↩︎

3. contains information about where to place different parts of the program in memory. ↩︎