UNIX for Advanced & Expert Users

Hi Experts,

If I understand correctly in posix standard system calls read/write or pread/pwrite a file descriptor should be sent.
How it works in case of raw disk? Are there something like files on raw devices? Or these disks are read just using block coordinates?

I want to understand how OS works with a disk without filesystem and with filesystem. Is there good document?

A raw disk is accessed through a file descriptor so is technically still a (single and big) file, as far as POSIX is concerned. There are operations that won't work like truncating or enlarging the file but otherwise, it can be read by seeking to the expected location and reading the data there. Same for writing.

Point: With modern hardware a "raw disc" can be multiple physical discs which the Operating System sees as one physical disc.

Some research ideas:

Consider the case of a virgin disc. There are no filesystems unless your software creates a filesystem. You can write or read whatever you like within the physical dimensions of the disc.

The very basic filesystems follow the pattern of the original IBM tape format VOL1 HDR1.

For research purposes it is worth a look at CAFS (Content Addressable File Store). That's the way to use a raw disc! It's so much better than "Binary Tree", "Hash Random" or "Index Sequential" file systems when dealing with disorganised data.