Linux NBD (Network Block Device) info

nbdinfo.py

#!/usr/bin/env python3
# Show Linux NBD (Network Block Device) info
# Requires Python 3.7+
# Author: Jonathon Reinhart
# License: MIT
from dataclasses import dataclass
from pathlib import Path

class NotConnected(Exception):
    pass

def read_int_file(p, base=10):
    return int(p.read_text(), base=base)

def human_size(s):
    prefixes = ("", "Ki", "Mi", "Gi", "Ti", "Pi", "Ei")
    for pref in prefixes:
        if s < 1024:
            return f"{s} {pref}B"
        s /= 1024
    return f"{s} {pref}B"


def pid_comm(pid):
    p = Path(f'/proc/{pid}/comm')
    return p.read_text().strip()


def get_nbd_params():
    paramdir = Path('/sys/module/nbd/parameters/')
    results = {}
    for name in ('nbds_max', 'max_part'):
        results[name] = read_int_file(paramdir / name)
    return results


def get_qemu_nbd_image(pid):
    p = Path(f'/proc/{pid}/fd')
    files = [fd for fd in p.iterdir() if fd.is_file()]
    if len(files) == 1:
        # NOTE: We assume that qemu-nbd only has one normal file open
        return files[0].readlink()
    raise Exception("forky says I dunno")


@dataclass
class NbdInfo:
    pid: int
    size: int
    ro: bool

    def infostr(self):
        procdesc = self.get_procname()
        try:
            procdesc += " " + str(self.get_srcfile())
        except (PermissionError, NotImplementedError):
            pass
        s = f"PID {self.pid} ({procdesc}), {human_size(self.size)}, "
        s += "read-only" if self.ro else "read/write"
        return s

    def get_procname(self):
        return pid_comm(self.pid)

    def get_srcfile(self):
        if self.get_procname() == "qemu-nbd":
            return get_qemu_nbd_image(self.pid)
        raise NotImplementedError(f"Don't know how to get srcfile for {self.get_procname()}")


def get_nbd_info(name):
    p = Path('/sys/class/block') / name

    try:
        pid = read_int_file(p / "pid")
    except FileNotFoundError:
        raise NotConnected

    return NbdInfo(
            pid = pid,
            size = read_int_file(p / "size"),
            ro = bool(read_int_file(p / "ro")),
            )


def main():
    params = get_nbd_params()

    for i in range(params['nbds_max']):
        name = f'nbd{i}'

        try:
            info = get_nbd_info(name)
        except NotConnected:
            print(f"{name}: (not connected)")
            continue

        print(f"{name}: {info.infostr()}")


if __name__ == '__main__':
    main()

Install into /usr/local/bin/nbdinfo.

Example output:

$ sudo nbdinfo 
nbd0: (not connected)
nbd1: PID 286233 (qemu-nbd /tmp/somedisk.vmdk), 2.0 GiB, read-only
nbd2: (not connected)
nbd3: (not connected)
nbd4: (not connected)
nbd5: (not connected)
nbd6: (not connected)
nbd7: (not connected)
nbd8: (not connected)
nbd9: (not connected)
nbd10: (not connected)
nbd11: (not connected)
nbd12: (not connected)
nbd13: (not connected)
nbd14: (not connected)
nbd15: (not connected)