Re: [PATCH] wic: add --reserved-size wks option

[Ed Bartosh <ed.bartosh@linux.intel.com>]

On Tue, Oct 18, 2016 at 11:24:14AM +0200, Maciej Borzęcki wrote:
> On Tue, Oct 18, 2016 at 10:38 AM, Ed Bartosh <ed.bartosh@linux.intel.com> wrote:
> > On Tue, Oct 18, 2016 at 10:37:22AM +0200, Maciej Borzęcki wrote:
> >> On Tue, Oct 18, 2016 at 9:31 AM, Ed Bartosh <ed.bartosh@linux.intel.com> wrote:
> >> > On Mon, Oct 17, 2016 at 04:46:00PM +0200, Maciej Borzęcki wrote:
> >> >> On Mon, Oct 17, 2016 at 3:22 PM, Ed Bartosh <ed.bartosh@linux.intel.com> wrote:
> >> >> > Hi Maciej,
> >> >> >
> >> >> > There is already --size and --extra-space options.
> >> >> > Can we get the same or similar result by just using them? Do we really
> >> >> > need new option for similar purpose?
> >> >>
> >> >> --reserved-size serves a different purpose, it establishes an upper
> >> >> bound on the size of a partition during layout. Unlike
> >> >> --size/--extra-space does not depend on the size of the filesystem
> >> >> image.
> >> >>
> >> >> For instance, assume I'm creating an image for SD card/eMMC with a
> >> >> fixed partition layout (something simple: boot partition, primary &
> >> >> secondary rootfs partitions, some data partition). Because future
> >> >> system updates are delivered as filesystem image, I want to make sure
> >> >> that there is exactly xxx MBs for my current and future rootfs images
> >> >> (regardless of current image size). Neither --size nor --extra-space
> >> >> can do that. I could use, say `--size 200 --overhead-factor 1`, but
> >> >> this will needlessly create a 200MB rootfs image and if I happen to
> >> >> cross the 200MB boundary I will not get an error.
> >> >>
> >> >> I had a private patch that added --fixed-size to enforce --size, but
> >> >> that would still end up creating filesystem image to fill the whole
> >> >> space.
> >> > I didn't get the difference between enforcing partition size and below
> >> > implementation. Can you elaborate a bit?
> >>
> >> `--fixed-size` was something that I had added to my fork back in 2014,
> >> even before `--overhead-factor` came in. The problem is that depending
> >> on a project you might want to have more control over how partitions
> >> are laid out, or even need to have a fixed layout. Adding
> >> `--fixed-size` would had a similar effect to what `--overhead-factor
> >> 1` does right now. Combined with `--size` would ensure that rootfs is
> >> say, 200MB large. The downside was that wic would actually create a
> >> 200MB rootfs, something that is not really necessary. In fact, I only
> >> wanted to have 200MB gap so that I have some spare space for future
> >> updates (where update is just a rootfs image you dd to the partition).
> >>
> > Thanks for the explanations. Now I got it - reserved size is not a part
> > of partition, it's a gap between partitions.
> 
> I might have not been clear enough when explaining. It's not a gap,
> it's just a container of size --reserved-size listed in MBR/GPT.
> There's probably a filesystem inside but not necessarily.
> Graphically it looks as like this:
> 
>                          --reserved-size
>                       |----------------------|
>                       v                      v
>     +-----------------+----------------------+---------------------+
>     |..... stuff .....|xxxxxxxxxx            |..... stuff .........|
>     +-----------------+----------------------+---------------------+
>                       ^         ^            ^
>                       |---------|------------|
>                        --size    --extra-space
> 
> 
Ah, I'm wrong again. It's a partition size limit, but it's not necessary
formatted, right? It's only formatted if size == reserved_size.

> >
> > 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.

--
Regards,
Ed