Purpose of stacking block devices

[Raphael Pour]

Hello,

could someone explain me the purpose of stacking block devices on top of
each other?

A block device with driver file or qcow2 "is usually" stacked on top of
a block device with driver raw as stated in the docs[1].

The related sentence came with [2] within the batch [3]. I couldn't
figure out why this 'stacking' might be useful.

Thanks and kind regards,
Raphael


[1]  Driver-specific options for raw:
https://qemu.weilnetz.de/doc/latest/system/invocation.html#hxtool-1
[2] doc: Document driver-specific -blockdev option:
https://lists.nongnu.org/archive/html/qemu-devel/2017-06/msg05325.html
[3] [Qemu-devel] [PULL 00/61] Block layer patches:
https://lists.nongnu.org/archive/html/qemu-devel/2017-06/msg05319.html

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