126 lines
3.2 KiB
Markdown
126 lines
3.2 KiB
Markdown
---
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type: theoretical
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backlinks:
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- "[[Memory Management]]"
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---
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A file system consists of two parts
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- Collection of files
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- A directory structure -> provides information about all files in the system
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## File
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- Logical view -> the unit of storing data
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Files are mapped by the OS onto physical nonvolatile devices
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**Types:**
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- Data
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- Numeric
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- Character
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- Binary
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- Program
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**Attributes**:
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- Name
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- Identifier (unique number)
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- Type[^2]
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- Location -> pointer
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- Size
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- Protection (permissions)
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- Datetime and user id
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### Logical Definition
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- Named collection of related information
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- Files may have free form (text files) or can be rigidly formatted[^1]
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### Operations
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- Create
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- Write
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- Read
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- Seek (reposition within file)
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- Delete
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- Truncate - shorten or cut off by removing data from the end
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- Open (load to memory)
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- Close (unload)
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### Open files
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Tracked by an **open-file table**, counted by **file-open count**.
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In order to [avoid race conditions](Inter-Process%20Communication.md#Avoiding%20race%20conditions), we need to lock the files somehow.
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- **Shared lock** -> several processes can acquire concurrently, used for reads
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- **Exclusive lock** -> writer lock
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- Mandatory vs. advisory -> access is denied depending on locks held and requested vs. processes can find status of locks and decide what to do
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### Structure
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Could be many:
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- None
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- Simple record
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- Lines
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- Fixed length
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- Variable length
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- Complex
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- Formatted document
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- Relocatable load file [^3]
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## Directories
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Collection of nodes containing information about all files. Also resides on disk.
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**Operations**:
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- Search for a file
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- Create a file
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- Delete a file
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- List a directory
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- Rename a file
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- Traverse file system
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### Single level directory
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A single directory for all users.
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Clearly, we need unique names, which can become a problem real fast. That shit is gonna grow super big.
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### Two-level directory
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Users have different directories. In Linux -> `/home/user` is separate, allowing for the same file names. Linux, however, uses a multi-level:
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### Tree-Structured Directories
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- Efficient searching
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- Grouping
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- Absolute v. relative path
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### Acyclic-Graph
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Have shared subdirectories and files. Symlinks achieve this.
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## Symlinks
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**Hard** vs **Soft**. Hard is a literal copy of the file but keep the same inode info, while soft is just a pointer.
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>[!IMPORTANT]
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>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
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## Disk
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Can be subdivided into **partitions**.
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Disk/partition can be used **raw** (no file system) or can be **formatted**. The entity containing the file system is known as a volume.
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> [!NOTE]- Typical fs organization
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> 
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## Access lists and groups
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Read, write and execute.
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Three classes of users on Linux
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1. Owner -> 7 (Read Write Execute)
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2. Group -> 6 (RW)
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3. Public -> 1 (X)
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---
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[^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
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[^2]: Extension (.pdf, .txt) as opposed to format, which specifies the [grammar](Regular%20languages.md) of the file
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[^3]: contains information about where to place different parts of the program in memory.
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