This is a story about booting ganeti instances from kernel inside virtual machine.
We are using ganeti-instance-debootstrap to create instances and have disk with 4k
blocks:

# blockdev --getbsz /dev/md127 
4096

For this to work, we need msdos partition table which is configured with

root@r1u32:~# grep ^PARTITION_STYLE /etc/default/ganeti-instance-debootstrap 
PARTITION_STYLE="msdos"

This should create one partition, offset by 1M (using default):

root@r1u32:~# tail -8 /etc/default/ganeti-instance-debootstrap 

# PARTITION_ALIGNMENT: the alignment of the partitions in sectors
# (512B); this defaults to 1MiB to give grub enough space for
# embedding and for better alignment with modern media (HDDs and
# SSDs), feel free to increase it if your media has even bigger
# allocation blocks
PARTITION_ALIGNMENT=2048

This however doesn't work if we are using 4k blocks on device because
offset of first partition stored in old primary partition chs format won't
match with different blocksize.

Solution is to create new partition table after debootstrap (which needs
host view of partition table with 4k block) using loopback which has 512b
blocksize.

Patch for this is: deboostrap-4k-msdos.diff

This patch is submitted to upstream: https://github.com/ganeti/instance-debootstrap/pull/2