notes/docs/lectures/osc/17_file_systems3.md

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 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.

File Allocation Tables

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

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.