[OE-core] [PATCH] wic: add --reserved-size wks option

[Ed Bartosh]

On Tue, Oct 18, 2016 at 03:59:23PM +0200, Maciej Borzęcki wrote:
> >> >> >> > What's the advantage of creating unusable gap over creating partition of
> >> >> >> > the same size that can be used?
> >> >> >>
> >> >> >> Just convenience.
> >> >> >>
> >> >> > What's the convenience of having extra space on partition that can't be
> >> >> > used for data over having it formatted and used?
> >> >> >
> >> >> >> > Even if that space is not needed it doesn't harm to have it, does it?
> >> >> >>
> >> >> >> I have not seen any negative side effects.
> >> >> >>
> >> >> > I do. If user needs that reserved space it's impossible to get it without
> >> >> > reformatting partition. The free space exists, but can't be used.
> >> >>
> >> >> That's not the point and is not aligned with use case I'm trying to solve.
> >> >>
> >> >> My case is rather simple, I'm creating an image for SD card that is
> >> >> deployed in the field. In that particular case, there's a primary and
> >> >> a secondary (aka. active and inactive) rootfs partitions that are
> >> >> switched whenever a system update comes in. The update is a file
> >> >> system image that is copied over to the inactive partition, followed
> >> >> by a system reboot.
> >> >>
> >> >> What I need is the ability to set a certain size of a partition (say
> >> >> 100MB), regardless of current rootfs size (which may be, say 70MB).
> >> >> The remaining unused space sets an upper limit on how much the rootfs
> >> >> may grow in the future (in this example case, it's 30MB). RIght now
> >> >> the best I can do is to describe a partition like this: `part /
> >> >> --source rootfs --size 100MB --overhead-factor 1`, hoping that if
> >> >> rootfs grows beyond 100MB I will somehow still be able to catch that
> >> >> and that the future images remain size compatible.
> >> >>
> >> >> The resulting filesystem inside the partition is larger than what
> >> >> IMAGE_CMD (ex. IMAGE_CMD_ext4) would give me, because of explicit
> >> >> --size in kickstart. I would prefer to have something comparable in
> >> >> size just to avoid later surprises, what implies using defaults.
> >> >> However, using defaults, means that I cannot control the layout
> >> >> because it will likely change each time rootfs size gets changed.
> >> >> There is no `--fixed-size` or other option to enforce specific size.
> >> >>
> >> >> Summing up, a simple use case that cannot be currently solved using wic.
> >> >>
> >> >> BTW. actually we're missing an ability to enforce --size (i.e.
> >> >> --fixed-size?) and a method passing an explicit partition offset
> >> >> inside the disk image (something useful for `--source rawcopy
> >> >> --no-table` partitions, currently solved with `--align`).
> >> >>
> >> > I undertood the problem and I agree that wic doesn't provide a solution.
> >> >
> >> > However, instead of making unformatted space I'd propose to format it,
> >> > i.e. to have --max-size option that would confict with --size and
> >> > specify upper size limit for the partition. All partition will be
> >> > formatted and available for data. This is identical to --fixed-size option
> >> > you've described. This approach would solve the problem you're
> >> > addressing and it would also make additional space usable.
> >> >
> >> > I'd also suggest to rename --size to --min-size and make --size deprecated.
> >> >
> >> > Does this make sense to you?
> >>
> >> No strong opinions here, just that deprecating --size might current
> >> users uneasy.
> >>
> > By deprecting --size I didn't mean removing it completely. We can just
> > print a warning suggesting usage of other options.
> >
> >> Perhaps --max-size could be a boolean switch? We could just name it
> >> --fixed-size (bool, defaults to False), with semantics that if
> >> --fixed-size is provided, the partition will have size --size,
> >> occurrence of --overhead-factor or --extra-space will raise an error.
> >>
> > That would work too, but it looks a bit confusing to me to have 2 different
> > types of size-related options.
> 
> Ok, but now we would have --min-size (previously known as --size) and
> --size (or --max-size?). That's still 2 size related options plus a
> deprecation warning.
> 

I agree, it doesn't look good. Moreover --max-size doesn't make it
clear that partition will be this size. It rather suggests that partition
can be this size or less.

So, we have --size which is actually minimum partition size, we have
couple of options to extend partition size (--overhead-factor and
--extra-space), but we don't have ability to set upper limit for partition
size.

OK, let's agree on using --fixed-size(int)
Using --fixed-size with any of --size or --overhead-factor or --extra-space
options should raise ks parser error. If rootfs size is bigger than
--fixed-size wic should raise an error too.

Any other opinions?

--
Regards,
Ed