[Libguestfs] [PATCH 2/4] test-md-and-lvm-devices: work around RAID0 regression in Linux v3.14/v5.4

[Laszlo Ersek]

On 09/20/21 07:23, Laszlo Ersek wrote:
> The "test-md-and-lvm-devices" test case creates, among other things, a
> RAID0 array (md127) that spans two *differently sized* block devices
> (sda1: 20MB, lv0: 16MB).
> 
> In Linux v3.14, the layout of such arrays was changed incompatibly and
> undetectably. If an array were created with a pre-v3.14 kernel and
> assembled on a v3.14+ kernel, or vice versa, data could be corrupted.
> 
> In Linux v5.4, a mitigation was added, requiring the user to specify the
> layout version of such RAID0 arrays explicitly, as a module parameter. If
> the user fails to specify a layout version, the v5.4+ kernel refuses to
> assemble such arrays. This is why "test-md-and-lvm-devices" currently
> fails, with any v5.4+ appliance kernel.
> 
> Until we implement a more general solution (see the bugzilla link below),
> work around the issue by sizing sda1 and lv0 identically. For this,
> increase the size of sdb1 to 24MB: when one 4MB extent is spent on LVM
> metadata, the resultant lv0 size (20MB) will precisely match the size of
> sda1.
> 
> This workaround only affects sizes, and does not interfere with the
> original purpose of this test case, which is to test various *stackings*
> between disk partitions, software RAID (md), and LVM logical volumes.
> 
> Related: https://bugzilla.redhat.com/show_bug.cgi?id=2005485
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
> ---
>  tests/md/test-md-and-lvm-devices.sh | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/tests/md/test-md-and-lvm-devices.sh b/tests/md/test-md-and-lvm-devices.sh
> index 5e82e3a4ff69..54f1c3bfb3f5 100755
> --- a/tests/md/test-md-and-lvm-devices.sh
> +++ b/tests/md/test-md-and-lvm-devices.sh
> @@ -53,7 +53,7 @@ trap cleanup INT QUIT TERM EXIT
>  # sda2: 20M PV (vg1)
>  # sda3: 20M MD (md125)
>  #
> -# sdb1: 20M PV (vg0)
> +# sdb1: 24M PV (vg0) [*]
>  # sdb2: 20M PV (vg2)
>  # sdb3: 20M MD (md125)
>  #
> @@ -66,6 +66,9 @@ trap cleanup INT QUIT TERM EXIT
>  # vg3   : VG (md125)
>  # lv3   : LV (vg3)
>  #
> +# [*] The reason for making sdb1 4M larger than sda1 is that the LVM metadata
> +# will consume one 4MB extent, and we need lv0 to offer exactly as much space
> +# as sda1 does, for combining them in md127. Refer to RHBZ#2005485.
>  
>  guestfish <<EOF
>  # Add 2 empty disks
> @@ -79,9 +82,10 @@ part-add /dev/sda p 64 41023
>  part-add /dev/sda p 41024 81983
>  part-add /dev/sda p 81984 122943
>  part-init /dev/sdb mbr
> -part-add /dev/sdb p 64 41023
> -part-add /dev/sdb p 41024 81983
> -part-add /dev/sdb p 81984 122943
> +part-add /dev/sdb p 64 49215
> +part-add /dev/sdb p 49216 90175
> +part-add /dev/sdb p 90176 131135
> +

I've removed the stray empty line just above, and pushed the patch as
commit 24f98939ae0c.

Thanks
Laszlo

>  
>  # Create volume group and logical volume on sdb1
>  pvcreate /dev/sdb1
>