This is the 2nd or 3rd thread containing hanging btrfs send, with kernel 4.18.x. The subject of one is "btrfs send hung in pipe_wait" and the other I can't find at the moment. In that case though the hang is reproducible in 4.14.x and weirdly it only happens when a snapshot contains (perhaps many) reflinks. Scrub and check lowmem find nothing wrong.
I have snapshots with a few reflinks (cp --reflink and also deduplication), and I see maybe 15-30 second hangs where nothing is apparently happening (in top or iotop), but I'm also not seeing any blocked tasks or high CPU usage. Perhaps in my case it's just recovering quickly. Are there any kernel config options in "# Debug Lockups and Hangs" that might hint at what's going on? Some of these are enabled in Fedora debug kernels, which are built practically daily, e.g. right now the latest in the build system is 4.19.0-0.rc3.git2.1 - which translates to git 54eda9df17f3. Chris Murphy
