I started...
  btrfs filesystem defragment -v /some/directory
and this took (unexpectedly, for just 150M of data) a long time (minutes).

So I wondered whether I could see via "strace -p PID" what was going on with 
the process.

Immediately when strace tries to attach, the btrfs process dies with this error 
message:

ERROR: defrag failed on /some/directory - Resource temporarily unavailable

This is 100% reproducable.

The strace output looks like this:
Process 1402 attached
brk(0)                                  = 0x1468000
brk(0)                                  = 0x1468000
brk(0x1460000)                          = 0x1460000
brk(0)                                  = 0x1460000
close(3)                                = 0
write(2, "ERROR: defrag failed on /some/d"..., 84) = 84
write(1, "btrfs-progs v4.1\n", 17)      = 17
write(2, "total 1 failures\n", 17)      = 17
exit_group(1)                           = ?
+++ exited with 1 +++

This is the first time I see a prozess crash upon strace being attached -
is this intended behaviour? If so... why?

(Tried again without strace, "the btrfs filesystem defragment" command later
finished ok, albeight after a loong time.)

Regards,

Lutz Vieweg

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