Re: Install to or Recover RAID Array Subvolume Root?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-04-18 01:22, David Alcorn wrote:
> Debian's default installer (1) can not create a BTRFS raid array
> during installation, and (2) installs to the default subvol of the
> BTRFS target.  The default subvol is 5 (BTRFS root) unless (i) prior
> to installation a BTRFS file-system was created, (ii) the default
> subvol is set to something other than 5, and (iii) you do not format
> the installation target during install. This perspective benefits from
> https://aykevl.nl/2015/11/debian-btrfs-subvolume.
>
> Can Debian install BTRFS to a pre-existing BTRFS RAID 6 array?  If the
> array's default is set to ,say subvolid 257, can I install to array
> subvolume 257?  The goal is to install to a subvolume on the array
> without disturbing date on other array subvolumes.
This is really something you should be asking on the Debian mailing 
list, or on one of their IRC channels.   That said,  based on my limited 
knowledge of the current state of the Debian installer, I think it 
should work, but I"m not certain.  I don't really use Debian much 
anymore except on a couple of Raspberry Pi's at work, so I really can't 
give a much better answer (Ironically, limitations like this in the 
installer are part of why I switched to Gentoo).
>
> I erred and shutdown my NAS during a balance.  Grub lost track of my
> root.  Root was on RAID 6 array subvolid 257.  I can boot a different
> root from a USB flash drive but neither update-grub not install-grub
> sees my old root on array subvolid 257.  I am happy to either recover
> or lose array subvolid 257 but do not want to lose data on other array
> subvol's.  I prefer to have my root on the array rather than a flash
> drive.  The balance completed successfully after I booted from the
> flash drive.
>
> I am running a debian back-ported 4.4.0 kernel with btrfs-progs v4.4
> on both my flash drive and array subvolid 257.  Both installs are
> UEFI.
It's rather refreshing to see somebody using an up to date kernel and 
userspace for once.  The issue with GRUB is however likely an issue with 
GRUB itself though (or possibly with something it uses for detecting 
filesystems), which brings up the question: How recent is the version of 
GRUB you're trying to install?  I would not be surprised if their 
current support for BTRFS raid5 and raid6 is not particularly good, 
especially considering that their support for BTRFS raid1 was kind of 
shoddy at first (and it still has occasional issues, like not working 
right half the time if one of the devices is missing).