Re: mkfs.jffs2 min erase block size: 4 vs 8 KiB

[Artem Bityutskiy <dedekind1@gmail.com>]

On Fri, 2011-06-24 at 13:10 -0400, Mike Frysinger wrote:
> On Fri, Jun 24, 2011 at 08:07, Artem Bityutskiy wrote:
> > On Wed, 2011-06-22 at 01:02 -0400, Mike Frysinger wrote:
> >> the current mkfs.jffs2 util has in its --eraseblock handling:
> >> /* If it's less than 8KiB, they're not allowed */
> >> if (erase_block_size < 0x2000) {
> >>     fprintf(stderr, "Erase size 0x%x too small. Increasing to 8KiB minimum\n",
> >>         erase_block_size);
> >>     erase_block_size = 0x2000;
> >> }
> >>
> >> but i cant seem to find documentation on where this comes from.  it's
> >> not uncommon to have SPI flashes with 4KiB erase sectors, and removing
> >> this sanity check and allowing 4 KiB seems to produce an image that
> >> works OK.
> >>
> >> am i missing something ?  or should i send a patch to drop this check ?
> >
> > I do not believe JFFS2 is able to work with such small erase blocks. In
> > JFFS2 (and in UBIFS as well) an eraseblock is the basic garbage
> > collection unit, and nodes cannot cross eraseblock boundaries. And the
> > maximum node size is 4KiB of data plus few bytes of node header. So 4KiB
> > won't even fit the largest data node, e.g., containing uncompressible
> > data.
> >
> > Even 8KiB is bad, because the amount of wasted space will be large -
> > JFFS2 will waste the rest of the eraseblock if it does not fit the node.
> > So the smaller EB is, the more wasted space you have.
> 
> i'll send a patch then to mkfs.jffs2 with a little bit of info.  i
> couldnt find a FAQ on the mtd webpage that covered this.

You could even send a patch against the mtd-www.git repo and add a piece
of doc to the web site :-)

> > For SPI flashes the way out is to emulate larger eraseblock sizes on MTD
> > level. I'd recommend at least 32 KiB.
> 
> sorry, but emulate how ?

Well, I was thinking about teaching MTD to merge real PEBs and report
larger EB size up to JFFS2. Probably this could be done via an MTD
module parameter and/or an ioctl.

>   just tell jffs2 to use an eraseblock size of
> 32KiB when creating the fs ?

Yes, the other way is to teach JFFS2 to merge PEBs and work with
multiple of PEBs. In fact in the past it could do this, but that was
removed because of some issues, do not remember what.

The only complication I see is bad blocks. When you merge PEBs and one
of them is bad, then the good PEBs which are in this merge should be
treated as bad as well. But this should not be very difficult to do.

On the one hand, teaching JFFS2 deal with small PEBs seems to make more
sense, on the other hand, doing this on driver level has an advantage -
you do not have to modify the file-system and you automatically make UBI
benefit from this - UBI scan time goes down when eraseblocks become
larger.

-- 
Best Regards,
Artem Bityutskiy (Битюцкий Артём)