How to install Debian 9 Stretch on software RAID1 (no GRUB installed)?

Question

linux

Greetings! I am making a machine for Ceph. The system is installed on a software RAID 1 consisting of 3 SSDs. I make two "physical partitions for RAID" on each of the SSDs, I collect them into two RAID1 volumes. On the first partition / (on ext4), the second partition is xfs (in reality there will be 12 xfs partitions for OSD logs, but I'm still trying to just install the system, I'll do the breakdown later).
I put Debian 9, in text mode. At the final stage of the installation, it is proposed to choose where to install GRUB - I choose one of the SSDs (I tried to install it on everything - /dev/sdm, /dev/sdn, /dev/sdo) - I get an error

Failed to install GRUB on /dev/md.
Failed to execute command 'grub-install /dev/md'.
This is an unrecoverable error.

Here's what's in the console:

grub-installer: mdadm: /dev/md does not appear to be an md device
grub-installer: grub-installer: error:
grub-installer: ioctl RAID_VERSION errorL Inappropriate ioctl for device.
grub-installer: error: Running 'grub-install --force "/dev/md" failed.

Here is what cat /proc/mdstat outputs:

Personalities : [raid1]
md1 : active raid1 sdm5[0] sdo5[2] sdn5[1]
2050011728 blocks super 1.2 [3/3] [UUU]
md0 : active raid1 sdm1[0] sdo1[2] sdn1[1]
29279232 blocks super 1.2 [3/3] [UUU]
unused devices:

Answer 1

I figured it out by reading my own question more closely :)
Regardless of which disk I chose to install GRUB, the system for some reason tried to put it on / dev / md, which, of course, led to an error. Therefore, I simply selected the "Enter manually" menu item and entered /dev/sdm there. After that, the installation went as expected, the system is loaded. Here is such a squiggle.

Answer 2

grub-install /dev/sd{o,m,n}
Do not forget to put GRUB in RAID1 in MBR on all three disks.