[systemd-devel] Using btrfs subvolumes with systemd

[Chris Murphy]

On Wed, Jun 8, 2016 at 12:48 PM, Florent Peterschmitt
<florent at peterschmitt.fr> wrote:
> Hi,
>
> I have a recurrent problem at boot time, when mounting filesystems.
>
> First :
>
> o This problem was on systemd v229 and is still here with v230
> o I use BTRFS for everything
> o I have 6 subvolumes
> o I use a subvolume as root filesystem, then mount other subvolumes
> o I use Gentoo as main distribution.
>
> Here are some outputs:
>
> # btrfs subvolume list /
> ID 257 gen 90 top level 5 path archlinux
> ID 258 gen 4296 top level 5 path gentoo
> ID 259 gen 2715 top level 5 path lib.modules
> ID 260 gen 4225 top level 5 path SteamLibrary
> ID 261 gen 613 top level 5 path ubuntu
> ID 262 gen 2875 top level 5 path usr.src
>
> # cat /etc/fstab
> /dev/mapper/root / btrfs autodefrag,noatime,subvol=gentoo 0 0
> tmpfs /var/tmp/portage/ tmpfs defaults 0 0
> UUID=98c23f22-566e-4a17-b663-3c0ce781a897 /boot ext4 defaults,noatime,nodev 0 0
> /dev/mapper/root /lib64/modules btrfs autodefrag,noatime,subvol=lib.modules 0 0
> /dev/mapper/root /usr/src btrfs autodefrag,noatime,subvol=usr.src 0 0
> /dev/mapper/root /home/florent/SteamLibrary btrfs autodefrag,noatime,subvol=SteamLibrary 0 0
>
> /dev/cdrom /mnt/cdrom auto noauto,ro 0 0
> /dev/fd0 /mnt/floppy auto noauto 0 0
>
> As you can see, /dev/mapper/root is the btrfs filesystem, created with "mkfs.btrfs
> /dev/mapper/root", nothing special. Same for subvolumes.
>
>
>
> Here is the problem:
>
> Each time i boot, this error happen:
> # journalctl -xe
> [...]
> systemd[1]: dev-mapper-root.device: Job dev-mapper-root.device/start timed out.
> systemd[1]: Timed out waiting for device /dev/mapper/root.
> [...]
>
> After StartExecTimeout (reduced to 10s…), i'm dropped into an emergency shell. There, i fire "mount -a", then quit, and the system boots.
>
> But only gentoo does continue booting, with ArchLinux (v230 too), i'm simply dropped into emergency shell forever until i mask usr-src.mount and lib-modules.mount before quitting!
>
>
> Is this a miss-configuration or a problem with systemd's device handling?

I take it /dev/mapper/root is a dmcrypt device?

If you can add systemd.log_level=debug at boot time as a boot
parameter (remove the symlink modification so the boot fails as it has
been), mount -a, then post the output from 'journalctl -b -o
short-monotonic > journal.log' somewhere, that might provide a bit
more context.

That it works for gentoo and not arch makes me wonder if this is
actually an issue in the initramfs. You could also add rd.debug as a
boot parameter, but the combination of both debugs makes things super
slow and the journal output will be quite large.


-- 
Chris Murphy