Messages in this thread | | | Subject | Re: partially encrypted filesystem | From | David Woodhouse <> | Date | Mon, 08 Dec 2003 14:07:33 +0000 |
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On Mon, 2003-12-08 at 13:44 +0000, phillip@lougher.demon.co.uk wrote: > dwmw2@infradead.org 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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