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docs/lectures/osc/17_file_systems3.md

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+## 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 before hand (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 straight forward - although **more seek operations** 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](/lectures/osc/assets/b8.png)
+
+##### File Allocation Tables
+
+* Store the linked-list pointers in a **separate index table** called a **file allocation table** in memory.
+
+![FAT](/lectures/osc/assets/b9.png)
+
+> **Advantages**
+>
+> * **Block size remains 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 200GB disk, with 1KB block size, 200 million entries are required, assuming that each entry at the table occupies 4 bytes, this required 800MB of main memory.
+
+#### I-Nodes
+
+Each file has a small data structure (on disk) called an **I-node** (index-node) that contains it's 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](/lectures/osc/assets/c1.png)