Bug#885575: md: potentially incorrect warnings about alignment

[Daniel Pocock <daniel@pocock.pro>]

Package: linux
Version: 4.13.13-1~bpo9+1

Looking at the output of dmesg, I see warnings like this:


md0: Warning: Device sde3 is misaligned


Checking the same disk with parted tells me it is aligned correctly:


# parted /dev/sde
GNU Parted 3.2
Using /dev/sde
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) align-check optimal 3                                           
3 aligned



Checking the disk with "fdisk -l" shows me:


# fdisk -l /dev/sde

Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes



When the same disk is attached through a USB HDD dock, the "fdisk -l"
output gives a different value for optimal I/O size:


# fdisk -l /dev/sdb

Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 33553920 bytes

=> 33553920 / 512 = 65535 sectors of 512 bytes


When I partitioned the disk, I used MiB alignment (multiple of 2048
sectors of 512 bytes each)

Is the warning from the md kernel module misleading or is parted
align-check wrong?  Can the warning be made more verbose to indicate why
it believes the partition is not aligned correctly?