Mental Model
A file system relies on data structures about the files, as opposed to the contents of that file. The former are called metadata—data that describes data. Each file is associated with an inode, which is identified by an integer, often referred to as an i-number or inode number.
- What on-disk structures store the file system’s data and metadata?
- What happens when a process opens a file?
- Which on-disk structures are accessed during a read or write?
How
Bitmap
When mounting a file system, the operating system will read the superblock first, to initialize various parameters, and then attach the volume to the file-system tree. When files within the volume are accessed, the system will thus know exactly where to look for the needed on-disk structures.
Directory
Often, file systems treat directories as a special type of file. Thus, a directory has an inode, somewhere in the inode table (with the type field of the inode marked as “directory” instead of “regular file”). The directory has data blocks pointed to by the inode (and perhaps, indirect blocks); these data blocks live in the data block region of our simple file system. Our on-disk structure thus remains unchanged.
Cache
Some applications (such as databases) don’t enjoy this trade-off. Thus, to avoid unexpected data loss due to write buffering, they simply force writes to disk, by calling fsync(), by using direct O interfaces that work around the cache, or by using the raw disk interface and avoiding the file system altogether.