[RFC] Writing sysupgrade.tgz directly to overlay for block-device sysupgrade

Jo-Philipp Wich jo at mein.io
Fri Jul 24 05:41:18 EDT 2020

Hi David,

> Instead of using the overlay filesystem du jour, perhaps it makes more
> sense to simply put the tarball itself, raw, on the loop device
> immediately after the end of the squashfs?

I suppose the limiting factor for the maximum backup size would be the amount
of available RAM then? Because somehow the tarball has to be kept somewhere
while it is overwritten due to filesystem formatting in order to unpack it
into the newly created fs afterwards.

What about writing the raw tarball to the *end* of the block device? Then the
new overlay FS could be formatted into the free space between squash and the
raw tarball, then the tarball could be extracted into the new fs, then fs
could be resized to occupy the entire space after squashfs till the end of the
block device.

That would allow backups to have at most 50% of the remaining free space after
squashfs as maximum size...


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