26/11/20

## File System Implementation

1. Contiguous
2. Linked Lists
3. File Allocation Table (FAT)
4. I-nodes (lookups)

### File access

Files will be composed of a number of blocks. Files are **sequential** or **random access**. Random access is essential for example in database systems.

#### Contiguous Allocation

**Contiguous file systems** are similar to **dynamic partitioning** in memory allocation.

> Each file is stored in a single group of **adjacent blocks** on the hard disk
>
> Allocation of free space can be done using **first fit, best fit, next fit**.
>
> - However when files are removed, this can lead to external fragmentation.
>
> **Advantages**
>
> - **Simple** to implement - only location of the first block and the length of the file must be stored
> - **Optimal read/write performance** - blocks are clustered in nearby sectors, hence the seek time (of the hard drive) is minimised
>
> **Disadvantages**
>
> - The **exact size** is not known beforehand (what if the file size exceeds the initially allocated disk space)
> - **Allocation algorithms** needed to decide which free blocks to allocate to a given file
> - Deleting a file results in **external fragmentation**
>
> Contiguous allocation is still in use in **CD-ROMS & DVDs**
>
> - External fragmentation isn't an issue here as files are written once.

#### Linked List Allocation

To avoid external fragmentation, files are stored in **separate blocks** that are **linked**.

> Only the address of the first block has to be stored to locate a file
>
> - Each block contains a **data pointer** to the next block
>
> **Advantages**
>
> - Easy to maintain (only the first block needs to be maintained in directory entry)
> - File sizes can **grow and shrink dynamically**
> - There is **no external fragmentation** - every possible block/sector is used (can be used)
> - Sequential access is straightforward - although **more seek operations** are required
>
> **Disadvantages**
>
> - **Random access is very slow**, to retrieve a block in the middle, one has to walk through the list from the start
> - There is some **internal fragmentation** - on average the last half of the block is left unused
>     - Internal fragmentation will reduce for **smaller block sizes**
>     - However, **larger blocks** will be **faster**
> - Space for data is lost within the blocks due to the pointer. The data in a **block is no longer a power of 2**
> - **Diminished reliability**: if one block is corrupted/lost, access to the rest of the file is lost.

![Linked list file storage](assets/b8.png)

##### File Allocation Tables

- Store the linked-list pointers in a **separate index table** called a **file allocation table** in memory.

![FAT](assets/b9.png)

> **Advantages**
>
> - **Block size remains a power of 2** - no more space is lost to the pointer
> - **Index table** can be kept in memory allowing fast non-sequential access
>
> **Disadvantages**
>
> - The size of the file allocation table grows with the number of blocks, and hence the size of the disk
> - For a 200 GB disk with a 1 KB block size, 200 million entries are required. Assuming that each entry in the table occupies 4 bytes, this requires 800 MB of main memory.

#### I-Nodes

Each file has a small data structure (on disk) called an **I-node** (index-node) that contains its attributes and block pointers.

> In contrast to FAT, an I-node is **only loaded when a file is open**
>
> If every I-node consists of $n$ bytes, and at most $k$ files can be open at any point in time, at most $n\times k$ bytes of main memory are required.

I-nodes are composed of **direct block pointers** (usually 10) **indirect block pointers** or a combination thereof.

![I-nodes](assets/c1.png)