File Systems
Filesystems are the machine's way of ordering your data on readable and/or writable media. They provide a logical way to access the stuff that you have down on disk so that you can read or modify extit. Which file system you use depends upon what you want to do with it. For example, Windows uses the Fat32 or NTFS filesystem. If your disk is really huge, then there's no point using Fat32 because the FAT system was designed in the days when nobody had disks as big as we do now. At the same time, there's no point using a NTFS filesystem on a tiny disk, because it was designed to work with large volumes of data - the overhead would be pointless for, say, reading a 1.44m floppy disk.
File System Theory
Inodes
Inodes (information nodes) are a crucial design element in most Unix filesystems: Each file is made of data blocks (the sectors that contains your raw data bits), index blocks (containing pointers to data blocks so that you know which sector is the nth in the sequence), and one inode block.
The inode is the root of the index blocks, and can also be the sole index block if the file is small enough. Moreover, as unix filesystems support hard links (the same file may appear several times in the directory tree), inodes are a natural place to store metadata such as file size, owner, creation/access/modification times, locks, etc.
Workings of File Systems
There are several common approaches to storing disk information. However, in comparison to memory management, there are some key differences in managing disk media:
- Data can only be written in fixed size chunks.
- Access times are different for different locations on the disk. Seeking is usually a costly operation.
- Data throughput is very small compared to RAM
- Data commonly has to be maintained
Hence some file systems have specialized structures, algorithms, or combinations thereof to improve speed ratings.
For details on specific filesystems, browse this list of filesystems
Allocation Table
The allocation table is comparable to the bitmap approach. However instead of just having a field free or occupied, it may contain other information. Advantage is that the use of this structure is simple, Disadvantage is that this approach is relatively slow, and a separate set of data is needed to define the which sections are used, and in which order. FAT for example, combines a linked list and an allocation table in the same structure.
Separate file and system areas
Some filesystems keep metadata and actual contents in separate areas on the disk. This makes specific sorts of data easy to find, as well as allowing for a special 'free' area. Downside is that you have to keep track of the boundary or boundaries between these areas, as the usage can differ and the disk has to adapt for that (big files: more data, less metadata; small files: less data, more metadata). This method is used prominently in SFS.
Other methods
Network File Systems
All these file systems are a way to create a large, distributed storage system from a collection of "back end" systems. That means you cannot (for instance) format a disk in 'NFS' but you instead mount a 'virtual' NFS partition that will reflect what's on another machine. Note that a new generation of File Systems is under heavy research, basing on latest P2P, cryptography and error correction techniques (such as the Ocean Store Project or Archival Intermemory.)
For details on various network file systems look here
File systems for OSDevers
There are many different kinds of filesystems around, from the well-known to the more obscure ones. The most unfortunate thing about filesystems is that every hobbyist OS programmer thinks that the filesystem they design is the ultimate technology, when in reality it's usually just a bad copy of DOS FAT with a change here and there. The world doesn't need another crap filesystem. Investigate all the possibilites before you decide you roll your own.