running on btrfs causes hang and unstartable instances with 4.3.20 and 4.3.18

[krichter]

Running on btrfs (with 4.3.20 and 4.3.18 on Linux 3.18.1, 3.17.6, 3.17.4, 3.16.7, 3.16.0 on Ubuntu 14.10 amd64) causes hangs of VMs (experienced with Ubuntu 15.04-daily amd64, Fedora 21 x86_64 and Fedora 21 i386 guests) at some point which require the VM process to be killed. After this kill the instance is no longer startable. The "Starting VM" dialog with progress bar stagnates at 0 % and after ~1 minute a second "Starting VM" dialog is displayed which stagnates at 20 %. This state stays for > 1 hour while 20 MB/s is constantly read from disk (on a system which usually reads data between 5 and 120 MB/s). Moving VirtualBox VMs to an ext4 filesystem (on a file image mounted via a loop device residing on the same btrfs) avoids this behavior. After 1 hour I usually kill the VM process.

The root system is btrfs and the VirtualBox VMs folder (usually created in the home directory is symlinked to another btrfs partition). Find attached `VBox.log of a hanging start on btrfs and a successful start on ext4 file image of the very same instance of Fedora 21 i386 unchanged between the two starts/start attempts.

The issue has been reproduced with 4.3.18 and 4.3.20 after ensuring that host kernel modules were installed correctly with both dkms autoinstall and /etc/init.d/vboxdrv setup.

I cannot test on a test build because the MD5 checksum validation fails after downloads with three different download managers. I suspect systematic failure which I can address in a separte issue, on the mailing list or in the forum if it is absolutely necessary.

[krichter]

VBox.log of a failing start of Fedora 21 i386 guest

[krichter]

VBox.log of a successful start of Fedora 21 i386 guest on an ext4 file image (log saved after successful boot)

[Frank Mehnert]

Most likely a duplicate of #11862. Make sure to remove the COW flags of all virtual disk image files.

[krichter]

Most likely a duplicate of #11862.

I agree.

Make sure to remove the COW flags of all virtual disk image files.

I'll wait until the warning dialog requested in #11862 is implemented, follow the instructions there and see if that fixes the issue.