Q: Filesystem choice..

[Eric W. Biederman]

David Woodhouse <dwmw2 at infradead.org> writes:

> On Mon, 2004-01-26 at 00:09 -0700, Eric W. Biederman wrote:
> > Has anyone gotten as far as a proof.  Or are there some informal
> > things that almost make up a proof, so I could get a feel?  Reserving
> > more than a single erase block is going to be hard to swallow for such
> > a small filesystem. 
> 
> You need to have enough space to let garbage collection make progress.
> Which means it has to be able to GC a whole erase block into space
> elsewhere, then erase it. That's basically one block you require.
> 
> Except you have to account for write errors or power cycles during a GC
> write, wasting some of your free space. You have to account for the
> possibility that what started off as a single 4KiB node in the original
> block now hits the end of the new erase block and is split between that
> and the start of another, so effectively it grew because it has an extra
> node header now. And of course when you do that you get worse
> compression ratios too, since 2KiB blocks compress less effectively than
> 4KiB blocks do.

Compression is an interesting question.  Do you encode the uncompressed
size of a block in bytes.  If so I don't think it would be too difficult
to get your uncompressed block size > page size.  With the page cache
there is real reason a block size <= page size.  You just need what
amounts to scatter/gather support.

My real question here is how difficult is it to disable compression?
Or can compression be deliberately disabled on a per file basis?

For the two primary files I am thinking of using neither one would
need compression.  A file of my BIOS settings is would be dense
and quite small (128 bytes on a big day).  A kernel is already
compressed and carries it's own decompresser, and whole file compression
is more effective than compressing small blocks.

> When you get down to the kind of sizes you're talking about, I suspect
> we need to be thinking in bytes rather than blocks -- because there
> isn't just one threshold; there's many, of which three are particularly
> relevant:

That makes sense.  This at least looks like a viable alternative for
the 1MB case.

[snip actual formulas]

> You want resv_blocks_write to be larger than resv_blocks_deletion, and I
> suspect you could get away with values of 2 and 1.5 respectively, if we
> were counting bytes rather than whole eraseblocks.

I have a truly perverse case I would like to ask your opinion about.
A filesystem composed of 2 8K erase blocks?  That is one of the
weird special cases that flash chips often support.  I could
only store my parameter file in there but it would be interesting.

I think if I counted bytes very carefully and never got above .5 of
a block full I suspect that it would work, and be useful.  I'd just
have to make certain the degenerate case matched the original jffs.

And a last question. jffs2 rounds all erase blocks up to a common size
doesn't it?

Eric