Bug 66702

Summary:	g_udev_client_query_by_device_number fails on disks with subvolumes
Product:	systemd	Reporter:	Jussi Pakkanen <jpakkane>
Component:	general	Assignee:	systemd-bugs
Status:	RESOLVED NOTABUG	QA Contact:	systemd-bugs
Severity:	normal
Priority:	medium
Version:	unspecified
Hardware:	Other
OS:	All
Whiteboard:
i915 platform:		i915 features:
Attachments:	Source code to demonstrate the issue

Description Jussi Pakkanen 2013-07-08 14:48:43 UTC

Created attachment 82191 [details]
Source code to demonstrate the issue

Sorry if this bug is filed in the wrong place. Information on GUdev is sparse and as far as I can tell, systemd is the upstream.

Anyway, the g_udev_client_query_by_device_number function fails on directories that are in subvolumes. An example is a btrfs file system with two subvolumes, @ (mounted as root) and @home (mounted as /home). This also happens when using encrypted home directories.

Having a subvolume might not be the real reason it fails, but that is what seems to be triggering the issue.

To replicate, download the attached source code. Compile it with "gcc -Wall -o idnum idnum.c `pkg-config --cflags --libs glib-2.0 gio-2.0 gudev-1.0`".

When run on a plain ext4 volume, the output is this:

./idnum
Device id is 2065.

Whereas on btrfs the output is this:

./idnum
Device id is 25.
Could not get device from num.

Originally filed here: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1197796

Comment 1 Kay Sievers 2013-07-08 20:48:00 UTC

Btrfs volumes are like NFS or tmpfs, a superblock without any associated
device. The behavior is expected and intentional.

Btrfs volumes might be constructed of many volumes, there cannot be the
concept of *that* device, like the "dumb" filesystems use.

There is no way for the udev API to find out which device is behind
a btrfs volume. Btrfs specific command need to be used to determine that,
no generic interface exists for that task.

Comment 2 Jussi Pakkanen 2013-07-09 07:28:01 UTC

Would it be possible for you to add a comment to the reference documentation about this so other people don't have to spend time with this issue? The page on GUdevClient does not mention subvolumes.

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