Re: [PATCH 1/1] ubi: Introduce block devices for UBI volumes

[Willy Tarreau <w@1wt.eu>]

On Sun, Feb 09, 2014 at 12:13:11AM +0100, Richard Weinberger wrote:
> Am 09.02.2014 00:01, schrieb Willy Tarreau:
> > On Sat, Feb 08, 2014 at 11:56:02PM +0100, Richard Weinberger wrote:
> >> Am 08.02.2014 23:51, schrieb Willy Tarreau:
> >>> On Sat, Feb 08, 2014 at 10:37:19PM +0100, Richard Weinberger wrote:
> >>>>> +config MTD_UBI_BLOCK_WRITE_SUPPORT
> >>>>> +       bool "Enable write support (DANGEROUS)"
> >>>>> +       default n
> >>>>> +       depends on MTD_UBI_BLOCK
> >>>>> +       select MTD_UBI_BLOCK_CACHED
> >>>>> +       help
> >>>>> +          This is a *very* dangerous feature. Using a regular block-oriented
> >>>>> +          filesystem might impact heavily on a flash device wear.
> >>>>> +          Use with extreme caution.
> >>>>> +
> >>>>> +          If in doubt, say "N".
> >>>>
> >>>> I really vote for dropping write support at all.
> >>>
> >>> Why ? When you put a read-only filesystem there such as squashfs, the
> >>> only writes you'll have will be updates, and write support will be the
> >>> only way to update the filesystem. So removing write support seriously
> >>> impacts the usefulness of the feature itself.
> >>
> >> So almost everyone has to enable MTD_UBI_BLOCK_WRITE_SUPPORT?
> >> I thought there is another way to fill the volume with data...
> > 
> > I personally don't see the use of disabling write support on anything
> > unless the code is broken. Better emit a warning upon first write to
> > mention that there is limited or no wear leveling. But preventing all
> > reasonable users from using a useful feature just to save a few ignorant
> > from shooting themselves in the foot is non-sense in my opinion.
> 
> As Piergiorgio wrote, one can use ubiupdatevol to update his squashfs.
> There is simply no use case for MTD_UBI_BLOCK_WRITE_SUPPORT.

I gave an example with ext2 for the config. It's a bit excessive to
quickly declare "there is simply no use case for $put_your_option_here",
it just means that *you* don't have this use case, which I perfectly
respect.

> > Why not disable write support to ubifs as well then, so that we're
> > sure that the most demanding ones will never wear their NANDs ? And
> > why not disable mtdblock so that nobody can mount them as ext2 ? If
> > people can already do bad things more easily without this code,
> > there is no reason to remove the feature.
> 
> I'd like to avoid another mtdblock.

But we already have it. Without write support on ubiblock, people will
continue to use mtdblock for this, which is even worse since it does
not handle bad blocks.

Regards,
Willy