Check which network block devices are in use

Question

How can I tell if a /dev/nbd* device is in use? Will "lsof" tell me? Or is there a better way? Is there an equivalent to "losetup -a" which will tell me the next free one?

Answer 1

From the man page:

nbd-client -c /dev/nbdX

Answer 2

nbd-client -c /dev/nbdX does not do the job correctly.

It may tell, that /dev/nbdX is not in use while it still is used internally in the kernel driver. In that case you neither see this in dmsetup nor with nbd-client.

Example:

% nbd-client -c /dev/ndb2; echo $?
1
% e2fsck /dev/nbd2; echo $?
e2fsck 1.42.13 (17-May-2015)
/dev/nbd2 is in use.
e2fsck: Cannot continue, aborting.
8

Actually the second indicator you need is revealed by fsck:

% strace e2fsck /dev/nbd2 2>&1 | grep open | grep /dev/nbd2
open("/dev/nbd2", O_RDONLY|O_EXCL) = -1 EBUSY (Device or resource busy)

Compare this with a really unused NBD-device:

% nbd-client -c /dev/ndb1; echo $?; strace e2fsck /dev/nbd1 2>&1 | grep open | grep /dev/nbd1
1
open("/dev/nbd1", O_RDONLY|O_EXCL) = 4

So this is the correct way to determine a /dev/nbdX is not used:

nbd-available()
{
nbd-client -c "$1";
[ 1 = $? ] &&
python -c 'import os,sys; os.open(sys.argv[1], os.O_EXCL)' "$1" 2>/dev/null;
}
get-available-nbd()
{
local a;
for a in /dev/nbd[0-9] /dev/nbd[1-9][0-9];
do
  nbd-available "$a" || continue;
  echo "$a";
  return;
done
return 1;
}

use it like nbd="`get-available-nbd`" || fail "no free NBD device"