vault backup: 2025-05-05 15:05:48

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Operating Systems/File Systems Management.md

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+---
+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
+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.