Understanding the concept of Inodes

Question

I am referring to the link about concepts of Inodes

I am confused on parts:

1. 12 direct block pointers
2. 1 single indirect block pointer
3. 1 double indirect block pointer
4. 1 triple indirect block pointer

Now the diagram says that each pointer is 32/64 bits.

• [Query]: Why and how are these values inferred? I mean why specifically have only 32 or 64 bit pointers?

The diagram says, One data block {8 KB} for each pointer {4 bytes/8 bytes}

• [Query]: How does this actually work out? i.e. 8*1024 bytes / 8 bytes = 1024 bytes? What is the logic behind having a 8 bytes pointer for 8KB block?

Answer 1

Sample calculation of maximum file size

* Assume that there are 10 direct pointers to data blocks, 1 indirect pointer, 1 double indirect pointer, and 1 triple indirect pointer
* Assume that the size of the data blocks is 1024 bytes = 1Kb, i.e., BlockSize = 1Kb
* Assume that the block numbers are represented as 4 byte unsigned integers, i.e., BlockNumberSize = 4b
* Some data blocks are used as index blocks. They store 1024 bytes / 4 bytes/entry = 256 entries
* Maximum number of bytes addressed by 10 direct pointers is

    = Number of direct pointers * Blocksize 
    = 10 * 1Kb
    = 10Kb


* Maximum number of bytes addressed by single indirect pointer is

    = NumberOfEntries * BlockSize
    = (Blocksize / BlockNumberSize) * BlockSize
    = (1Kb / 4b) * 1Kb
    = 256 * 1Kb
    = 256Kb


* Maximum number of bytes addressed by double indirect pointer is

    = NumberOfEntries^2 * BlockSize
    = (Blocksize / BlockNumberSize)^2 * BlockSize
    = (1Kb / 4b)^2 * 1Kb
    = (2^10 / 2^2)^2 * (2^10b)
    = (2^8)^2 * (2^10)b
    = (2^16) * (2^10)b
    = 2^6 * 2^20 b
    = 64 Mb


* Maximum number of bytes addressed by triple indirect pointer is

    = NumberOfEntries^3 * BlockSize
    = (Blocksize / BlockNumberSize)^3 * BlockSize
    = (1Kb / 4b)^3 * 1Kb
    = (2^10 / 2^2)^3 * (2^10b)
    = (2^8)^3 * (2^10)b
    = (2^24) * (2^10)b
    = 2^4 * 2^30 b
    = 16 Gb


* Maximum file size is 16Gb + 64Mb + 266Kb