<!-- !> Notes from Ferd's class 
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# File Systems

- different ways of storing persistend data
- file systems - organize and store data on block devices

- 2 main abstractions
  1. File
     - collection of bytes associated with a name
  2. Directories
     - collection of directories and files


## File (System) APIs

Basic ones
- open - open a file
- close - close a file
- read/write - read or write from or to an open file

Others:
- lseek - adjust the offset of the given file
- fsync - flush buffers by writing to the disk
- rename - (atomically) rename a file
- (f)stat - get file metadata
- link - create a new name and associate it with file data, that is,
  create a hard link
- unlink - remove a name (label) from file data - if number of links
  drops to 0, this means "delete the file"

Directories:
- mkdir
- rmdir
- opendir / closedir
- readdir


## Filesystem Implementation

- implemented as a device driver
  - file system then communicates with the block device driver

- 2 major tasks of a file system:
  1. Translate names + offsets to the actual data/storage
  2. Free space management

- design of a FS
  1. Data structs: contents, metadata
  2. Access methods

## Useful observations about a typicial FS

- Most files: relatively small
  - ~2K most common size
- Average size: growin
  - ~200K 
- Most bytes occupied by large files
- Relatively large number of files
  - ~100K
- File systems - roughly half full
- Directories are relatively small
  - most directories will have ~20 files or less

# Organization

- a disk is divided blocks
- typical size 4K = 4096B
- 1M drive = 256 blocks
- blocks numbered 0 to N - 1 (for a disk partition size of 4K * N)
- let's say we only have 32 blocks 
- here's how we might split the disk up:

      +---------------------------------------------------------------+
      |S|i|d|I|I|I|I|I|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|D|
      +---------------------------------------------------------------+
      0 1 2 3 4 5 6 7       11      15                              31

- We have the following regions
  + S: superblock
  + i/d: free space bitmap for I blocks and D blocks
  + I: metadata stored as i-nodes
  + D: data blocks - actual file contents

## I-nodes

- metadata about the file
- the disk contains a table of these
- simply an array of structs stored on the disk
- fixed size - for easy searching
  - find file using it's i-node number (i-node number * sizeof(inode_t))
- Fields (example):
  1. size - long int - 8 bytes
  2. Owner - short int - 2 bytes
  3. Group - short int - 2 bytes
  4. Access - short int - 2 bytes
  5. Modification timestamp - long int - 8 bytes
  6. Refernce count - short int - 2 bytes
  7. Blocks - array of char - 8 bytes


## Directories

- To get from filename -> inode: directories
- A directory is a specially flagged i-node and a data block containing a
  table of filenames associated with i-node numbers
- Root directory of the file system: a designated i-node (e.g., i-node 0)


### Example
- example: file.txt - size 2K (occupies one block)

How to get from `/home/ferd/file.txt` -> i-node 42

1. go to i-node 0 (/)

2. find its datablock:

       home   d   3
       usr    d   2

3. go to i-node 3 (home)

4. find its datablock:

       ferd   d   7
       bob    d   5
       alice  d   12

5. go to i-node 7 (ferd)

6. find its datablock

       file.txt  f  42
       test.csv  f  55

7. go to-inode 42