Re: partially encrypted filesystem

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > Of course, all this is at the logical file level, and ignores the 
> > physical blocks on disk.  All filesystems assume physical data blocks 
> > can be updated in place.  With compression it is possible a new physical 
> > block has to be found, especially if blocks are highly packed and not 
> > aligned to block boundaries.  I expect this is at least partially why 
> > JFFS2 is a log structured filesystem.
> 
> Not really. JFFS2 is a log structured file system because it's designed
> to work on _flash_, not on block devices. You have an eraseblock size of
> typically 64KiB, you can clear bits in that 'block' all you like till
> they're all gone or you're bored, then you have to erase it back to all
> 0xFF again and start over.
> 
> Even if you were going to admit to having a block size of 64KiB to the
> layers above you, you just can't _do_ atomic replacement of blocks,
> which is required for normal file systems to operate correctly.

Are those assumptions needed for something else than recovery after
crash/powerdown? [i.e., afaics 64K ext2 should work on flash, but fsck
might have some troubles...]

								Pavel
-- 
When do you have a heart between your knees?
[Johanka's followup: and *two* hearts?]