Access Paths: Reading and Writing
This lesson explains the step by step process of vsfs reading and writing a file to and from a disk.
We'll cover the following...
Now that we have some idea of how files and directories are stored on disk, we should be able to follow the flow of operation during the activity of reading or writing a file. Understanding what happens on this access path is thus the second key in developing an understanding of how a file system works; pay attention!
For the following examples, let us assume that the file system has been mounted and thus that the superblock is already in memory. Everything else (i.e., inodes, directories) is still on the disk.
Reading a file from disk
In this simple example, let us first assume that you want to simply open a file (e.g., /foo/bar), read it, and then close it. For this simple example, let’s assume the file is just 12KB in size (i.e., 3 blocks).
When you issue an open("/foo/bar", O_RDONLY) call, the file system first needs to find the inode for the file bar, to obtain some basic information about the file (permissions information, file size, etc.). To do so, the file system must be able to find the inode, but all it has right now is the full pathname. The file system must ...