On 15.01.2005, at 17:49, Peter da Silva wrote:
Journaling is the process of the file system writing changes to disk
during low processor demand times, so that if your system crashes, your
file system, if it had time to write any changes, is not damaged.
Virtually all file systems do this, UFS has worked this way for at least two decades.
A Journal is different, it's a change log. It's on disk, and 'dirty' blocks are written to sequentially, rather than updated back to the disk block they came from. When a read occurs, the journal is checked before (or instead of) the conventional file system. There's three basic ways this can operate:
First, the journal is a log of the in-memory cached blocks. When they're finally flushed, the journal is cleaned and re-used. This is the easiest way to add journalling to an existing file system.
Second, the journal is a primary store. A cleaner process runs through the journal and copies the blocks to the final location in the background, new blocks can then be written into the space the cleaner has freed up.
Third, the journal is the whole store. There's nothing on the disk but the journal, there's no conventional file system under it. This is pretty rare, the Sprite logging file system and the Network Appliance WAFL do this.
I'm glad journaling is now permanent part of the sys - I still use the same sys installation since developer versions of Panther came out - and I had some ruff crashes on my main machine due to a bad ram stick and some other hardware issues
also I made my Jaguar machines journaling with some diskutil commands - no major trouble there as well
hth Sven
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