Re: partially encrypted filesystem

[David Woodhouse]

On Mon, 2003-12-08 at 13:44 +0000, phillip@xxxxxxxxxxxxxxxxxxx wrote:
> dwmw2@xxxxxxxxxxxxx wrote:
> > On Sat, 2003-12-06 at 00:50 +0000, Phillip Lougher wrote:
> > > 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.
> 
> Curiously, I am aware of how flash and log structured filesystems work.

This I assumed. The explanation was more for the benefit of the peanut
gallery than yourself.

> > JFFS2 was designed to avoid that inefficient extra layer, and work
> > directly on the flash. Since overwriting stuff in-place is so difficult,
> > or requires a whole new translation layer to map 'logical' addresses to
> > physical addresses, it was decided just to ditch the idea that physical
> > locality actually means _anything_.
> 
> Maybe okay for a flash filesystem which is slow anyway, but many
> filesystem designers *are* concerned about physical locality of
> blocks, for example video filesystems.

Oh, absolutely. 

> Or maybe 'not in(to)-place' :-)

:)

>  I don't think I was saying compression is difficult, it is not
> difficult if you've designed the filesystem correctly.

My point was that it's trivial in JFFS2 not because I designed the file
system 'correctly', but mostly because of other factors which just
happened to lead to a design which, by coincidence, made compression
trivial.

-- 
dwmw2

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