Filesystem crash recovery
Filesystem writes have two major components to them. At the bottom level, you are writing out blocks of data to the disk. In addition, there is some amount of filesystem metadata involved too. Examples of metadata include the directory tree, the list of blocks and attributes associated with each file, and the list of what blocks on disk are free.
Like many disk-oriented activities, filesystems have a very clear performance vs. reliability trade-off they need to make. The usual reliability concern is what happens in the situation where you're writing changes to a file and the power goes out in the middle.
Consider the case where you're writing out a new block to a file, one that makes the file bigger (rather than overwriting an existing block). You might do that in the following order:
- Write data block.
- Write file metadata referencing use of that block.
- Add data block to the list of used space metadata.
What happens if power goes out between steps...
