Re: Debian Bullseye install btrfs raid1

[richard lucassen <mailinglists@lucassen.org>]

On Wed, 4 May 2022 14:06:31 +0200
Hans van Kranenburg <hans@knorrie.org> wrote:

> > Still new to btrfs, I try to set up a system that is capable of
> > booting even if one of the two disks is removed or broken. The BIOS
> > supports this.
> > 
> > As the Debian installer is not capable of installing btrfs raid1, I 
> > installed Bullseye using /dev/md0 for /boot (ext2) and a / btrfs on
> > /dev/sda3. This works of course. After install I added /dev/sdb3 to
> > the / fs: OK.
> 
> Did you 'just' add the disk to the filesystem, or did you also do a
> next step of converting the existing data to the raid1 profile?

AFAIK this is what I need to do to convert sda3 mounted on / to a
raid1 using sda3/sdb3:

btrfs device add /dev/sdb3 /
btrfs balance start -dconvert=raid1 -mconvert=raid1 /

> If you start out with 1 disk and simply add another, it tells btrfs
> that it can continue writing just 1 (!) copy of your data wherever it 
> likes. And, in this case, the filesystem *always* wants (needs!) all 
> disks to be present to mount, of course.
> 
> disk 1  disk 2
> A       C
> B       E
> D
> 
> If you want everything duplicated on both disks, you need to convert
> the existing data that you already had on the first disk to the raid1 
> profile, and from then on, it will keep writing 2 copies of the data
> on any two disks in the filesystem (but you have exactly 2, so it's
> always on both of those two in that case).
> 
> disk 1  disk 2
> A       D
> B       B
> D       C
> C       A
> 
> If the previous installed system still works well when you add back
> the second disk again, you can still do this. (so, when you did not
> force any destructive operations, and just had it fail like seen
> below)
> 
> Can you share output of the following commands:
> 
> btrfs fi usage <mountpoint>
> 
> With the following command you let it convert all (d)ata and
> (m)etadata to the raid1 profile:
> 
> btrfs balance start -dconvert=raid1 -mconvert=raid1 /

That's what I did

> Afterwards, you can check the result with the usage command. The
> data, metadata, and system lines in the output of the usage command
> should all say RAID1, and you should see that on both disks, a
> similar amount of data is present.

Just chose the grub boot option "Debian 11 on sda6" (an older install),
this works but in fact this seems to be the sda3/sdb3 raid1. I must have
messed up grub somewhere:

btrfs filesystem show
Label: none  uuid: f9cf579f-d3d9-49b2-ab0d-ba258e9df3d8
        Total devices 2 FS bytes used 1.15GiB
        devid    1 size 16.00GiB used 2.28GiB path /dev/sda3
        devid    2 size 16.00GiB used 2.28GiB path /dev/sdb3

Label: none  uuid: 1739f989-05e0-48d8-b99a-67f91c18c892
        Total devices 2 FS bytes used 448.00KiB
        devid    1 size 16.00GiB used 2.57GiB path /dev/sda5
        devid    2 size 16.00GiB used 2.56GiB path /dev/sdb5

Label: 'data'  uuid: 3173a224-830f-41d7-8870-3db0e8c986c9
        Total devices 2 FS bytes used 1020.38MiB
        devid    1 size 187.32GiB used 2.01GiB path /dev/sda6
        devid    2 size 187.32GiB used 2.01GiB path /dev/sdb6

I will first clean up all the mess I created.

"There are two types of people: those who have lost data and those who
will" :-)

R.

-- 
richard lucassen
https://contact.xaq.nl/