Windows and Linux software Raid dual boot BIOS machine

One could think that nowadays having a machine with software raid doing dual boot should be easy, but... my experience showed that it is not that easy.

Having a Windows machine do software raid is easy (I still don't understand why it doesn't really work like it should, but that is because I'm used to Linux software raid), and having software raid on Linux is also really easy. But doing so on a BIOS booted machine, on mbr disks (as Windows doesn't allow GPT on BIOS) is quite a pain.

The problem is how Windows does all this, with it's dynamic disks. What happens with this is that you get from a partitioning like this:

/dev/sda1 * 2048 206847 204800 100M 7 HPFS/NTFS/exFAT /dev/sda2 206848 312580095 312373248 149G 7 HPFS/NTFS/exFAT /dev/sda3 312580096 313165823 585728 286M 83 Linux /dev/sda4 313165824 957698047 644532224 307,3G fd Linux raid autodetect

To something like this:

/dev/sda1 63 2047 1985 992,5K 42 SFS /dev/sda2 * 2048 206847 204800 100M 42 SFS /dev/sda3 206848 312580095 312373248 149G 42 SFS /dev/sda4 312580096 976769006 664188911 316,7G 42 SFS

These are the physical partitions as seen by fdisk, logical partitions are still like before, of course, so there is no problem in accesing them under Linux or windows, but what happens here is that Windows is using the first sectors for its dynamic disks stuff, so... you cannot use those to write grub info there :-(

So... the solution I found here was to install Debian's mbr and make it boot grub, but then... where do I store grub's info?, well, to do this I'm using a btrfs /boot which is on partition 3, as btrfs has room for embedding grub's info, and I setup the software raid with ext4 on partition 4, like you can see on my first partition dump. Of course, you can have just btrfs with its own software raid, then you don't need the fourth partition or anything.

There are however some caveats on doing all this, what I found was that I had to install grub manually using grub-install --no-floppy on /dev/sda3 and /dev/sdb3, as Debian's grub refused to give me the option to install there, also... several warnings came as a result, but things work ok anyway.

One more warning, I did all this on Buster, but it looks like for Grub 2.04 which is included on Bullseye, things have gotten a bit bigger, so at least on my partitions there was no room for it, so I had to leave the old Buster's grub around for now, if anybody has any ideas on how to solve this... they are welcome.