I will try to explain journaling as I see it. I probably will not give the process completely right, but you will have a better idea of how journaling works.
Let us say I want to write some new data to the journal file system. The steps would be something like the following.
- The operating system writes a new entry to the journal. This entry
states the I want to write the new data. The operating system also
includes what every information would be needed to undo the
- The operating system marks in the journal that it finished making
the journal entry.
- The operating system now goes through the process of writing the new
data to the drive.
- The operating system marks in the journal that it has
completed the task stated in the journal entry. The space used in
the journal by this entry can now be reused.
If the system crashes during these steps, then the following happens during restart of he computer.
If the computer never completed step 2, then the journal entry can me marked completed so the space can be reused. In other words, the task of writing the data was never started, so the journal entry can be ignored.
If the computer completed step 2, but not step 4, then the information stored in the journal entry is used to return the file system to the state before the journal entry was made. After this is completed, the journal entry can be marked as completed so the space can be reused.
So journaling is a temporary operation that helps prevent the corruption of a file system. This slows down access to the drive, usually this cost is outweighed by the benefit of the protection.
Time machine backups are a permanent backup of changes made to your drive. You can use the backup to restore parts or all of your file system to a previous state.