*** Description of problem:

The "tests/md/test-md-and-lvm-devices.sh" script of the test suite kept failing for me; I looked into it. Unfortunately, it's a horrible complication for libguestfs (this is why this BZ is being filed as an RFE).

The log file contains the following lines:

> test-md-and-lvm-devices.sh: info: you can skip this test by setting SKIP_TEST_MD_AND_LVM_DEVICES_SH=1
> *stdin*:22: libguestfs: error: md_create: mdadm: md-sda1-lv0: mdadm: Defaulting to version 1.2 metadata
> mdadm: RUN_ARRAY failed: Unknown error 524
> FAIL md/test-md-and-lvm-devices.sh (exit status: 1)

A web search promptly leads to the following references:
- https://www.linuxquestions.org/questions/linux-general-1/raid-arrays-not-assembling-4175662774/
- https://blog.icod.de/2019/10/10/caution-kernel-5-3-4-and-raid0-default_layout/
- https://github.com/torvalds/linux/commit/c84a1372df929033cb1a0441fb57bd3932f39ac9

My understanding is the following.

- In Linux 3.14, a regression was introduced, whereby the layout of a RAID0 array of disks of *different sizes* changed incompatibly. Assembling an old layout array with a new kernel, or a new layout array with an old kernel, would lead to data corruption.

- There is no way to query the layout of an existent array. A read-only mis-assembly would not cause data corruption, but it cannot be reliably used to probe the existent layout.

- To mitigate the mess, Linux 5.4 introduced a module parameter called "raid0.default_layout", where the user is *required* to specify the layout of the RAID0 arrays. If Linux detects a RAID0 array of disks of different sizes, it will not guess, but reject the assembly, unless the user explicitly sets the layout via the module parameter.

- This is a module parameter, so it governs all RAID0 arrays. There is no way to specify different layouts for different arrays.

Consequences for libguestfs:

- If the guest array was created with a kernel >= 3.14, but the appliance uses a kernel < 3.14, the appliance will corrupt the array.

- If the guest's RAID0 array of disks of different sizes was created with a kernel < 3.14, and the appliance uses a kernel in [3.14, 5.4), the appliance will corrupt the array.

- Because of the above two points, libguestfs should reject starting with an appliance kernel < 5.4.

- If the appliance uses a kernel >= 5.4, then the libguestfs application (or end-user, in case of guestfish) must be *forced* with some new parameter to set the RAID0 layout. This layout parameter then has to be passed to the appliance kernel when the appliance is launched.

- The "test-md-and-lvm-devices.sh" test case needs to be updated. In the short term, it should be modified so that the block devices *directly constituting* any given array have precisely the same size -- this should activate the (conf->nr_strip_zones == 1) branch in the appliance kernel, and evade the problem.

- In the long term, based on the above requirement that the appliance kernel be >= 5.4 (even for array creation), a new test case using different disk sizes for RAID0 should be added, and it should explicitly pass "raid0.default_layout=2" to the appliance kernel (using the new libguestfs / guestfish parameter).

*** Version-Release number of selected component (if applicable):
Upstream f47e0bb67254 ("appliance: reorder mounting of special filesystems in init", 2021-09-15).

*** How reproducible:
100%

*** Steps to Reproduce:
1. make check