"grub-install: error: unaligned device size." When grub-install is run from a bcache-backed filesystem.

[J Mo]

Hello!

I am trying to do a drive migration and have encountered an error that 
I've never seen before, and has me confused:

address@hidden grub-install /dev/disk/by-id/ata-WDC_WD3003FZEX-xxxxxxxxxxxx
Installing for i386-pc platform.
grub-install: error: unaligned device size.



Googling for "grub-install: error: unaligned device size." came up with 
nothing at all.

I would assume grub is complaining about some partition being unaligned, 
but that's not the case as far as I can see.

Environment here is a typical PC, Debian Unstable with grub packages 
2.02~beta3-5 and standard Debian kernel 4.9.0-2-amd64.

The key factor causing this problems seems to be that I am running 
grub-install from a chroot environment on top of a bcache-backed filesystem.

Note that when I talk about bcache here, I'm talking about the old 
bcache type, not the new bcachefs.

If I copy my grub boot directory to another non-bcache filesystem 
location, it works fine.

I tried various overrides like --force and -s without success.

Here's what the end of grub-install with -v looks like:

grub-install: info: copying `/usr/share/grub/unicode.pf2' -> 
`/boot/grub/fonts/unicode.pf2'.
grub-install: info: /dev/bcache0 is not present.
grub-install: info: Looking for /dev/bcache0.
grub-install: info: /dev/bcache0 is a parent of /dev/bcache0.
grub-install: info: /dev/bcache0 is present.
grub-install: info: Looking for /dev/bcache0.
grub-install: info: /dev/bcache0 is a parent of /dev/bcache0.
grub-install: info: /dev/bcache0 is present.
grub-install: info: Looking for /dev/bcache0.
grub-install: info: /dev/bcache0 is a parent of /dev/bcache0.
grub-install: info: drive = 0.
grub-install: error: unaligned device size.



I am guessing that the grub-install process is doing some kind of sanity 
check to make sure that the /boot part is going to be accessible to the 
stage1 bootloader found in the MBR/bios-boot-part. The info messages 
above seem to indicate that grub-install trips up recursively on the 
bcache device. In this case, I was using a /boot grub install that was 
on a bcache-backed ext4 filesystem.

My workaround is to just copy the files to a temporary non-bcached 
location and run "grub-install --boot-directory=/mnt/new-bcache-boot/ 
/dev/disk/by-id/ata-WDC_WD3003FZEX-xxxxxxxxxxx", but it seems kinda 
silly that I need to do this.

Please advise.