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cannot boot with BTRFS in degraded mode

From: Giovanni Biscuolo
Subject: cannot boot with BTRFS in degraded mode
Date: Wed, 04 Sep 2019 16:36:03 +0200

Hi Guix!

Yesterday I had to physically replace a failed disk on milano-guix-1
(one of Guix build machines), that disk was part of a BTRFS RAID10
multi disk array and now the machine is unbootable

The BTRFS RAID10 array was made of 6 disks and was running well, some
days ago Christopher Baines found that the 5th disk (/dev/sde) of that
array failed and was able to remount it in degraded mode in order to
re-balance the array and go on working without data loss

Unfortunately I was not able to perform a "btrfs replace..."  since
adding a new disk (we have spare slots) was not detected by the
kernel... HP ProLiant Smart Array is not so smart after all (aka bye bye
hot swapping of disks) :-S...

So I had to reboot the server and enter the config tool, added the new
drive as a new Smart Array logical volume (RAID0 with 1 drive) [1] and
removed the failed logical volume

The problem now is that the boot process stops when trying to mount the
BTRFS filesystem, the error is:

--8<---------------cut here---------------start------------->8---
BTRFS error (device sda3): devid 5 uuid [omissis] is missing
--8<---------------cut here---------------end--------------->8---

([omissis] means I'm not copying the exact uuid, sda3 is the first block
device in the BTRFS pool)

All I get now is the guix rescue environment prompt, that I do not know
how to use: I'm not able to boot with BRTFS in degraded mode :-S

Christopher suggested I might be able to at least mount the filesystem
with the degraded option in the guix rescue environment, which might be
something like:

--8<---------------cut here---------------start------------->8---
         (mkdir "/mnt/broken-root")
         (mount "/dev/sda3" "/mnt/broken-root" "btrfs" 0 "degraded")
--8<---------------cut here---------------end--------------->8---

but we do not know how to proceed from there.

Obviously I have no way now to reconfigure guix, the only idea I got is
to boot from an USB rescue disk (e.g. grml) and try to do a "btrfs
replace..." from there: that should fix the BTRFS array and should allow
a mount in non-degraded mode, so the next Guix boot should succeed

That machine is physically far away from me and I should collect as much
info as possible before I go there to test for a solution (no remote
serial console unfortunately)

I'm searching the web for a solution, any hint will be greatly
appreciated :-)

Meanwhile milano-guix-1 build machine is offline... :-(

Thank you for your attention, Gio'

[1] AFAIU that is the only way to present a single disk to the OS and
let the OS manage it as part of a **software** RAID pool (hardware RAID
is not an option)

Giovanni Biscuolo

Xelera IT Infrastructures

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