Re: [Buildroot] [RFC PATCH 1/2] support/scripts/genimage.sh: support creating a bmap image

[Thomas Petazzoni via buildroot <buildroot@buildroot.org>]

Hello Peter,

On Wed, 28 Aug 2024 15:58:21 +0200
Peter Korsgaard <peter@korsgaard.com> wrote:

>  > I understand your concern, but compression is also what helps in making
>  > those sparse images small.  
> 
> NIT: No, it is the sparseness (E.G. the holes). Compression in fact
> breaks the holes (replaces them by zeros):

You didn't understand my point, and Arnout explained it better on the
chat.

My point is that it is *very* easy to lose the sparseness. You copy the
file around, you share it in Google Drive, or download it over HTTP or
what not, and bang the sparseness is lost.

Instead, with a compressed file, you get a file that has pretty much
the same size as the sparse file, but you can move it around, transfer
it, it won't consume its real size. And the bmap metadata file allows
to use/flash that compressed file with the same efficiency as the
original sparse file.

In fact, bmap is quite useless is the file sparseness is retained:
instead of using bmap metadata, you could just ask what are the holes
of the file and skip them when flashing. So really the whole point of
bmap metadata IMO is that they allow to do fast flashing (skipping
holes) even if the sparseness of the file has been lost.

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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