This commit is contained in:
John Gatward
173
docs/lectures/osc/16_file_systems2.md
Normal file
173
docs/lectures/osc/16_file_systems2.md
Normal file
@@ -0,0 +1,173 @@
|
||||
20/11/20
|
||||
|
||||
## File System Views
|
||||
|
||||
### User View
|
||||
|
||||
A **user view** that defines a file system in terms of the **abstractions** that the operating system provides
|
||||
|
||||
An **implementation view** that defined the file system in terms of its **low level implementation**
|
||||
|
||||
**Important aspects of the user view**
|
||||
|
||||
> * The **file abstraction** which **hides** implementation details from the user
|
||||
> * File **naming policies**, user file **attributes** (size, protection, owner etc)
|
||||
> * There are also **system attributes** for files (e.g. non-human readable, archive flag, temp flag)
|
||||
> * **Directory structures** and organisation
|
||||
> * **System calls** to interact with the file system
|
||||
>
|
||||
> The user view defines how the file system looks to regular users and relates to **abstractions**.
|
||||
|
||||
#### File Types
|
||||
|
||||
Many OS's support several types of file. Both windows and Unix have regular files and directories:
|
||||
|
||||
* **Regular files** contain user data in **ASCII** or **binary** format
|
||||
* **Directories** group files together (but are files on an implementation level)
|
||||
|
||||
Unix also has character and block special files:
|
||||
|
||||
* **Character special files** are used to model **serial I/O devices** (keyboards, printers etc)
|
||||
* **Block special files** are used to model drives
|
||||
|
||||
### System Calls
|
||||
|
||||
File Control Blocks (FCBs) are kernel data structures (they are protected and only accessible in kernel mode)
|
||||
|
||||
* Allowing user applications to access them directly could compromise their integrity
|
||||
* System calls enable a **user application** to **ask the OS** to carry out an action on it's behalf (in kernel mode)
|
||||
* There are **two different categories** of **system calls**
|
||||
* **File manipulation**: `open()`, `close()`, `read()`, `write()` ...
|
||||
* **Directory manipulation**: `create()`, `delete()`, `rename()`, `link()` ...
|
||||
|
||||
### File Structures
|
||||
|
||||
**Single level**: all files are in the same directory (good enough for basic consumer electronics)
|
||||
|
||||
**Two or multiple level directories**: tree structures
|
||||
|
||||
* **Absolute path name**: from the root of the file system
|
||||
* **Relative path name**: the current working directory is used as the starting point
|
||||
|
||||
**Directed acyclic graph (DAG)**: allows files to be shared (links files or sub-directories) but **cycles are forbidden**
|
||||
|
||||
**Generic graph structure**: Links and cycles can exist.
|
||||
|
||||
The use of **DAG** and **generic graph structures** results in **significant complications** in the implementation
|
||||
|
||||
* Trees are a DAG with the restriction that a child can only have one parent and don't contain cycles.
|
||||
|
||||
When searching the file system:
|
||||
|
||||
* Cycles can result in **infinite loops**
|
||||
* Sub-trees can be **traversed multiple times**
|
||||
* Files have **multiple absolute file names**
|
||||
* Deleting files becomes a lot more complicated
|
||||
* Links may no longer point to a file
|
||||
* Inaccessible cycles may exist
|
||||
* A garbage collection scheme may be required to remove files that are no longer accessible from the file system tree.
|
||||
|
||||
#### Directory Implementations
|
||||
|
||||
Directories contain a list of **human readable file names** that are mapped onto **unique identifiers** and **disk locations**
|
||||
|
||||
* They provide a mapping of the logical file onto the physical location
|
||||
|
||||
Retrieving a file comes down to **searching the directory file** as fast as possible:
|
||||
|
||||
* A **simple random order of directory** entries might be insufficient (search time is linear as a function of the number of entries)
|
||||
* Indexes or **hash tables** can be used.
|
||||
* They can store all **file related attributes** (file name, disk address - Windows) or they can **contain a pointer** to the data structure that contains the details of the file (Unix)
|
||||
|
||||
|
||||
|
||||
##### System Calls
|
||||
|
||||
Similar to files, **directories** are manipulated using **system calls**
|
||||
|
||||
* `create/delete`: new directory is created/deleted.
|
||||
* `opendir, closeddir`: add/free directory to/from internal tables
|
||||
* `readdir`: return the next entry in the directory file
|
||||
|
||||
**Directories** are **special files** that **group files** together and of which the **structure is defined** by the **file system**
|
||||
|
||||
* A bit is set to indicate that they are directories
|
||||
* In Linux when you create a directory, two files are in that directory that the user has no control over. These files are represented as `.` and `..`
|
||||
* `.` - a file dealing with file permissions
|
||||
* `..` - represents the parent directory (`cd ..`)
|
||||
|
||||
##### Implementation
|
||||
|
||||
> Regardless of the type of file system, a number of **additional considerations** need to be made
|
||||
>
|
||||
> * **Disk Partitions**, **partition tables**, **boot sectors** etc
|
||||
> * Free **space management**
|
||||
> * System wide and per process **file tables**
|
||||
>
|
||||
> **Low level formatting** writes sectors to the disk
|
||||
>
|
||||
> **High level formatting** imposes a file system on top of this (using **blocks** that can cover multiple **sectors**)
|
||||
|
||||
### Partitions
|
||||
|
||||
Disks are usually divided into **multiple partitions**
|
||||
|
||||
* An independent file system may exist on each partiton
|
||||
|
||||
**Master Boot Record**
|
||||
|
||||
* Located as the start of the entire drive
|
||||
* Used to boot the computer (BIOS reads and executes MBR)
|
||||
* Contains **partition table** at its end with **active partition**.
|
||||
* One partition is listed as **active** containing a boot block to load the operating system.
|
||||
|
||||
|
||||
|
||||
#### Unix Partition
|
||||
|
||||
> The partition contains
|
||||
>
|
||||
> * The partition **boot block**:
|
||||
> * Contains code to boot the OS
|
||||
> * Every partition has a boot block - even if it does not contain an OS
|
||||
> * **Super block** contains the partitions details e.g. partition size, number of blocks, I-node table etc
|
||||
> * **Free space management** contains a bitmap or linked list that indicates the free blocks.
|
||||
> * A linked list of disk blocks (also known as grouping)
|
||||
> * We use free blocks to hold the **number of the free blocks**. Since the free list shrinks when the disk becomes full, this is not wasted space
|
||||
> * **Blocks are linked together**. The size of the list **grows with the size of the disk** and **shrinks with the size of the blocks**
|
||||
> * Linked lists can be modified by **keeping track of the number of consecutive free blocks** for each entry (known as counting)
|
||||
> * **I-Nodes**: An array of data structures, one per file, telling all about the files
|
||||
> * **Root directory**: the top of the file-system tree
|
||||
> * **Data**: files and directories
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
Free space management with linked list (on the left) and bitmaps (on the right)
|
||||
|
||||
**Bitmaps**
|
||||
|
||||
* Require extra space
|
||||
* Keeping it in main memory is possible but only for small disk
|
||||
|
||||
**Linked lists**
|
||||
|
||||
* No wasted disk space
|
||||
* We only need to keep in memory one block of pointers (load a new block when needed)
|
||||
|
||||
Apart from the free space memory tables, there is a number of key data structures stored in memory:
|
||||
|
||||
* An in-memory mount table (table with different partitions that have been mounted)
|
||||
* An in-memory directory cache of recently accessed directory information
|
||||
* A **system-wide open file table**, containing a copy of the FCB for every currently open file in the system, including location on disk, file size and **open count** (number of processes that use the file)
|
||||
* A **per-process open file table**, containing a pointer to the system open file table.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
Reference in New Issue
Block a user