File Systems

Depends on	Filing System

You should be reasonably happy about the principle of a filing system before proceeding with this article.

There is the filing system – the overall system whereby files (in general) are kept and then there can be a file system which is a component of the whole. This is probably most easily illustrated on a Windows system: a ‘memory stick’ contains its own file system which may or may not be implemented in the same way as (for example) the hard disk drive file system(s). In Windows it appears as a visibly separate device. Nevertheless files can easily be transferred back-and-forth.

Each volume may have its own file system. A volume is a logical drive. Typically a volume may correspond to a disk partition, although this is not mandatory. Having some awareness of the volume in which a file resides can sometimes be useful. For example, moving a file within a volume is probably just a matter of renaming it and, possibly, changing a couple of directory entries; moving a file between volumes involves copying all the bytes – and that can take a long time for a big file.

There are a number of choices already in existence - some of which are now somewhat obsolete. (Like many aspects of computing, the capacity of storage devices has increased enormously, over the years.)

Example: Linux ext2

This is just a sketch of a file system to give some idea as to a way it can be done. Some (actually, quite a few!) details are therefore omitted.

For the details of how the data blocks are assembled into files (in this system), consult the more detailed article on i-nodes.

• The superblock and group descriptor specify details like the size of the group, the number of i-node entries, the start of the free-block list and so forth.
• The bitmaps are a convenient way of indicating whether a data block/i-node is used or not.
• The i-node table is an array of records which defines both the file (data blocks) and its attributes.
• The data blocks are managed in some way which is (hopefully) efficient for access on the particular medium. For example, spaces may be left deliberately to reduce file fragmentation if a file grows gradually.
• A directory is just a file which contains a linked list of entries. File-names vary in length so space is allocated (and deallocated) in much the same way as RAM; this sometimes leaves ‘gaps’ (unused bytes) in the file.

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Also refer to:	Operating System Concepts, 10th Edition: Chapter 15.1, pages 597-598

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Articles on Filing System

"Everything is a File" • FAT • File Access • File Attributes • File Descriptor • File Locking • File Permissions • File Systems • File Types • Files • Filing System • Filing System Implementation • Fragmentation • I-nodes • Journalling File System • Links • Network File System (NFS) • Resources