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

From: "Gero Schwäricke via buildroot" <buildroot@buildroot.org>
To: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Cc: buildroot@buildroot.org
Subject: Re: [Buildroot] [RFC PATCH 1/2] support/scripts/genimage.sh: support creating a bmap image
Date: Wed, 24 Apr 2024 10:06:18 +0200	[thread overview]
Message-ID: <20240424080618.GB1370614@theia> (raw)
In-Reply-To: <20240423113313.GB527510@theia>

Hi all,

I have to correct myself:

On Tue, Apr 23, 2024 at 01:33:13PM +0200, Gero Schwäricke wrote:
> Hi Dario,
> 
> I tried your patch, but it didn't work for me. bmaptool failed with an
> error. The issue was that my home directory is encrypted using ecryptfs.
> This does not implement all ioctl options that are needed for bmaptool
> to read the block allocation of the image file. There also seem to be
> issues with ZFS, so I would recommend to not enable this by default for
> defconfigs.
> 
> A workaround for the issue described above would be to sparse copy the
> image to a mktemp file and run bmaptool on that. However that will only
> work if /tmp uses tmpfs (iiuc). And will potentially eat a lot of memory
> for this step.

This won't work. On second thought I wondered how cp can determine a
sparse file while bmaptool can't. So I checked coreutils, and iiuc,
`cp --sparse=always` will detect holes by reading the file and
generating holes for all blocks that contain only zeros. Copying a file
like this and then using bmaptool on the copy would destroy the benefit
of bmaptool. Because bmaptool can differentiate between blocks
containing zeroes and blocks that are empty. Empty blocks are holes (no
need to copy those), blocks containing zeroes are relevant (will be
copied).

Best,
Gero

> 
> I generally like the approach of bmaptool and think including this in
> buildroot seems like a good idea. Maybe the documentation should also
> include a section on that in "14.1 Boot the generated images". Currently
> it only covers NFS boot and Live CD. Different optimizations for sdcard
> flashing seem to be missing.
> 
> Best,
> Gero

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