92 lines
3.6 KiB
Markdown
92 lines
3.6 KiB
Markdown
26/11/20
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## File System Implementation
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1. Contiguous
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2. Linked Lists
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3. File Allocation Table (FAT)
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4. I-nodes (lookups)
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### File access
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Files will be composed of a number of blocks. Files are **sequential** or **random access**. Random access is essential for example in database systems.
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#### Contiguous Allocation
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**Contiguous file systems** are similar to **dynamic partitioning** in memory allocation.
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> Each file is stored in a single group of **adjacent blocks** on the hard disk
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> Allocation of free space can be done using **first fit, best fit, next fit**.
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> - However when files are removed, this can lead to external fragmentation.
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> **Advantages**
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> - **Simple** to implement - only location of the first block and the length of the file must be stored
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> - **Optimal read/write performance** - blocks are clustered in nearby sectors, hence the seek time (of the hard drive) is minimised
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> **Disadvantages**
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> - The **exact size** is not known beforehand (what if the file size exceeds the initially allocated disk space)
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> - **Allocation algorithms** needed to decide which free blocks to allocate to a given file
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> - Deleting a file results in **external fragmentation**
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> Contiguous allocation is still in use in **CD-ROMS & DVDs**
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> - External fragmentation isn't an issue here as files are written once.
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#### Linked List Allocation
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To avoid external fragmentation, files are stored in **separate blocks** that are **linked**.
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> Only the address of the first block has to be stored to locate a file
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> - Each block contains a **data pointer** to the next block
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> **Advantages**
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> - Easy to maintain (only the first block needs to be maintained in directory entry)
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> - File sizes can **grow and shrink dynamically**
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> - There is **no external fragmentation** - every possible block/sector is used (can be used)
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> - Sequential access is straightforward - although **more seek operations** are required
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> **Disadvantages**
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> - **Random access is very slow**, to retrieve a block in the middle, one has to walk through the list from the start
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> - There is some **internal fragmentation** - on average the last half of the block is left unused
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> - Internal fragmentation will reduce for **smaller block sizes**
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> - However, **larger blocks** will be **faster**
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> - Space for data is lost within the blocks due to the pointer. The data in a **block is no longer a power of 2**
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> - **Diminished reliability**: if one block is corrupted/lost, access to the rest of the file is lost.
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##### File Allocation Tables
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- Store the linked-list pointers in a **separate index table** called a **file allocation table** in memory.
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> **Advantages**
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> - **Block size remains a power of 2** - no more space is lost to the pointer
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> - **Index table** can be kept in memory allowing fast non-sequential access
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> **Disadvantages**
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> - The size of the file allocation table grows with the number of blocks, and hence the size of the disk
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> - 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.
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#### I-Nodes
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Each file has a small data structure (on disk) called an **I-node** (index-node) that contains its attributes and block pointers.
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> In contrast to FAT, an I-node is **only loaded when a file is open**
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> 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.
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I-nodes are composed of **direct block pointers** (usually 10) **indirect block pointers** or a combination thereof.
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