Kernel: 3.18.2 with btrfs builtin systemd git 72521ab rootfs of "host": ext4 Running systemd-nspawn -b -D /media/test-intra --template=/media/centos7-base fails with: Couldn't create snapshot /media/test-intra from /media/centos7-base: Inappropriate ioctl for device This looks to me like systemd-nspawn doesn't verify whether the FS providing -D is btrfs or not and just blindly tries to create a btrfs snapshot on top of it. IMHO this needs at least to be handled a bit more graceful - letting the user know what's the problem. Besides that, if there's no sane way to create a btrfs snapshot (e.g. using an image file) on top of another filesystem, this should be made clear in the systemd-nspawn manpage.
I am pretty sure we should issue the ioctl always, regardless what the backing fs is. Maybe one day the ioctls for snapshotting become generically used... We can certainly improve the log message though, and tell the use that he needs btrfs as source if we encounter ENOTTY here...
nspawn in current git now supports --template= on non-btrfs file systems, hence this bug is probably obsolete. Closing.
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