I am using Ubuntu 18.04 and have an md0 raid array of 4x500GB drives, ie. 2TB

I ran this command dd if=/dev/zero of=./swapfile bs=1MiB count=350KiB

Which I expected to create a 350GB file,

However the command is stuck, it has not returned and the file size is not longer growing, has been same for 20 minutes now.

$ ls -lrt
total 311956800
-rw-r--r-- 1 root root 319443763200 Apr  7 17:59 swapfile
$ df -k .
Filesystem      1K-blocks      Used  Available Use% Mounted on
/dev/md0       1953017856 262744128 1689747456  14% /mnt/nvme1

What is issue?

ps: I did this

$ strace -p 2318
strace: Process 2318 attached


There was no output, and I could not unattach... CTRL-C does not work.

  • do you have strace installed? see what it says. strace -p $(pidof dd) (or check the PID manually)
    – ilkkachu
    Commented Apr 7, 2021 at 17:35
  • # strace -p 2318 strace: Process 2318 attached (there is not other output)
    – guesty
    Commented Apr 7, 2021 at 17:38
  • that's all? Usually it at least shows e.g. read(0, if the process is blocking on a system call. Anyway, if there is more, you might want to edit it to the question, the formatting in comments isn't very good for printouts like that
    – ilkkachu
    Commented Apr 7, 2021 at 17:40
  • Which state is the dd process in (see ps -f 2318)?
    – fra-san
    Commented Apr 7, 2021 at 17:55

1 Answer 1


In case it helps someone else.... The filesystem was BTRFS (I probably should have mentioned that but it did not occur to me) -- I just formatted /dev/md0 as EXT4 -- and as if by magic everything started working. :-)

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