Re: btrfs as / filesystem in RAID1

[Hans van Kranenburg <Hans.van.Kranenburg@mendix.com>]

Hi Stefan,

On 2/1/19 11:28 AM, Stefan K wrote:
> 
> I've installed my Debian Stretch to have / on btrfs with raid1 on 2
> SSDs. Today I want test if it works, it works fine until the server
> is running and the SSD get broken and I can change this, but it looks
> like that it does not work if the SSD fails until restart. I got the
> error, that one of the Disks can't be read and I got a initramfs
> prompt, I expected that it still runs like mdraid and said something
> is missing.
> 
> My question is, is it possible to configure btrfs/fstab/grub that it
> still boot? (that is what I expected from a RAID1)

Yes. I'm not the expert in this area, but I see you haven't got a reply
today yet, so I'll try.

What you see happening is correct. This is the default behavior.

To be able to boot into your system with a missing disk, you can add...
    rootflags=degraded
...to the linux kernel command line by editing it on the fly when you
are in the GRUB menu.

This allows the filesystem to start in 'degraded' mode this one time.
The only thing you should be doing when the system is booted is have a
new disk present already in place and fix the btrfs situation. This
means things like cloning the partition table of the disk that's still
working, doing whatever else is needed in your situation and then
running btrfs replace to replace the missing disk with the new one, and
then making sure you don't have "single" block groups left (using btrfs
balance), which might have been created for new writes when the
filesystem was running in degraded mode.

-- 
Hans van Kranenburg