---
type: theoretical
backlinks:
  - "[[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)
- Type[^2]
- 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 formatted[^1]

### 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](Inter-Process%20Communication.md#Avoiding%20race%20conditions), 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
> ![](Pasted%20image%2020250505144352.png)


## 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)
 



---

[^1]: **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

[^2]: Extension (.pdf, .txt) as opposed to format, which specifies the [grammar](Regular%20languages.md) of the file

[^3]: contains information about where to place different parts of the program in memory.