Question

I am a unclear about file system implementation. Specifically (Operating Systems – Tannenbaum (Edition 3), Page 275) states “The first word of each block is used as a pointer to the next one. The rest of block is data”.

Can anyone please explain to me the hierarchy of the division here? Like, each disk partition contains blocks, blocks contain words? and so on…

Answer 1

I don’t know the context of this sentence, but it appears to be describing a linked list of blocks. Generally speaking, a “block” is a small number of bytes (usually a power of two). It might be 4096 bytes, it might be 512 bytes, it depends. Hard drives are designed to retrieve data a block at a time; if you want to get the 1234567th byte, you’ll have to get the entire block it’s in. A “word” is much smaller and refers to a single number. It may be as low as 2 bytes (16-bit) or as high as 8 bytes (64-bit); again, it depends on the filesystem.

Of course, blocks and words isn’t all there is to filesystems. Filesystems typically implement a B-tree of some sort to make lookups fast (it won’t have to search the whole filesystem to find a file, just walk down the tree). In a filesystem B-tree, each node is stored in a block. Many filesystems use a variant of the B-tree called a B+-tree, which connects the leaves together with links to make traversal faster. The structure described here might be describing the leaves of a B+-tree, or it might be describing a chain of blocks used to store a single large file.

In summary, a disk is like a giant array of bytes which can be broken down into words, which are usually 2-8 bytes, and blocks, which are usually 512-4096 bytes. There are other ways to break it down, such as heads, cylinders, sectors, etc.. On top of these primitives, higher-level index structures are implemented. By understanding the constraints a filesystem developer needs to satisfy (emulate a tree of files efficiently by storing/retrieving blocks at a time), filesystem design should be quite intuitive.