Hello linux-btrfs,

I'm trying to clone a subvolume with 'btrfs send' but it always hangs 
for hours.

I tested this on multiple systems. All showed the same result:
- Manjaro (btrfs-progs v4.17.1; linux v4.18.5-1-MANJARO)
- Ubuntu 18.04 in VirtualBox (btrfs-progs v4.15.1; linux v4.15.0-33-generic)
- ArchLinux in VirtualBox (btrfs-progs v4.17.1; linux v4.18.5-arch1-1-ARCH)
All following logs are from the ArchLinux VM.

To make sure it's not the 'btrfs receive' at fault I tried sending into 
a file using the following command:
'strace -o btrfs-send.strace btrfs send -vvv -f intenso_white.snapshot 
/mnt/intenso_white/@data/.snapshots/test-snapshot'

The 'btrfs send' process always copies around 1.2-1.4G of data, then 
stops all disk IO and fully loads one cpu core. btrfs scrub found 0 
errors. Neither did btrfsck. 'btrfs device stats' is all 0.

I would be thankful for all ideas and tips.

Regards
Stefan

---------------------------------------------

The btrfs-send.strace is attached. So is the dmesg.log during the hang.

Stack traces of the hung process:
--- /proc/3022/task/3022/stack ---
[<0>] 0xffffffffffffffff
--- /proc/3022/task/3023/stack ---
[<0>] pipe_wait+0x6c/0xb0
[<0>] splice_from_pipe_next.part.3+0x24/0xa0
[<0>] __splice_from_pipe+0x43/0x180
[<0>] splice_from_pipe+0x5d/0x90
[<0>] default_file_splice_write+0x15/0x20
[<0>] __se_sys_splice+0x31b/0x770
[<0>] do_syscall_64+0x5b/0x170
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[<0>] 0xffffffffffffffff

[vagrant@archlinux mnt]$ uname -a
Linux archlinux 4.18.5-arch1-1-ARCH #1 SMP PREEMPT Fri Aug 24 12:48:58 
UTC 2018 x86_64 GNU/Linux

[vagrant@archlinux mnt]$ btrfs --version
btrfs-progs v4.17.1

[vagrant@archlinux mnt]$ sudo btrfs fi show /dev/sdb1
Label: 'intenso_white'  uuid: 07bf61ed-7728-4151-a784-c4b840e343ed
Total devices 1 FS bytes used 655.82GiB
devid    1 size 911.51GiB used 703.09GiB path /dev/sdb1

[vagrant@archlinux mnt]$ sudo btrfs fi df /mnt/intenso_white/
Data, single: total=695.01GiB, used=653.69GiB
System, DUP: total=40.00MiB, used=96.00KiB
Metadata, DUP: total=4.00GiB, used=2.13GiB
GlobalReserve, single: total=512.00MiB, used=0.00B