nbdinfo - Man Page

display information and metadata about NBD servers and exports


 nbdinfo [--json] NBD

NBD is an NBD URI or subprocess:

 NBD := nbd://... | nbd+unix:// (or other URI formats)
      | [ CMD ARGS ... ]
 nbdinfo --size [--json] NBD
 nbdinfo --is read-only|rotational NBD
 nbdinfo --can cache|connect|... NBD
 nbdinfo --map [--totals] [--json] NBD
 nbdinfo -L|--list [--json] NBD
 nbdinfo --help
 nbdinfo --version


nbdinfo displays information and metadata about an NBD server.

The single required parameter can be the NBD URI of the server (see https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md):

 $ nbdinfo nbd://localhost
 protocol: newstyle-fixed without TLS, using structured packets
         export-size: 1048576 (1M)
         content: data
         uri: nbd://localhost:10809/
         is_rotational: false
         is_read_only: false
         can_cache: true
         can_df: true
         can_fast_zero: true
         can_flush: true
         can_fua: true
         can_multi_conn: true
         can_trim: true
         can_zero: true
         block_size_minimum: 1
         block_size_preferred: 4096
         block_size_maximum: 33554432

For an NBD server on a local Unix domain socket you would use a command such as this (with similar output to above):

 $ nbdinfo "nbd+unix:///?socket=/tmp/unixsock"

Or you can run the NBD server as a subprocess (see section "Subprocess" below):

 $ nbdinfo -- [ qemu-nbd -r -f qcow2 file.qcow2 ]

JSON output

To display the output as JSON (eg. for scripting with jq(1)) add the --json parameter:

 $ nbdinfo --json nbd://localhost | jq .
   "protocol": "newstyle-fixed",
   "TLS": false,
   "structured": true,
   "exports": [
       "export-name": "",
       "content": "DOS/MBR boot sector; partition 1 : ID=0xc, start-CHS (0x3ff,254,63), end-CHS (0x3ff,254,63), startsector 2048, 4148704 sectors",
       "uri": "nbd://localhost:10809/",
       "is_rotational": false,
       "is_read_only": true,
       "can_cache": true,
       "can_df": true,
       "can_fast_zero": false,
       "can_flush": false,
       "can_fua": false,
       "can_multi_conn": true,
       "can_trim": false,
       "can_zero": false,
       "block_size_minimum": 1,
       "block_size_preferred": 4096,
       "block_size_maximum": 33554432,
       "export-size": 2125119488,
       "export-size-str": "2075312K"


To display only the size in bytes of the NBD export (useful for scripting) use the --size parameter:

 $ nbdinfo --size nbd://localhost
 $ nbdinfo --size [ nbdkit null 1M ]

Test for flags

Use one of the options below to test NBD flags.  The command does not print anything.  Instead it exits with success (exit code 0) if true, or failure (exit code 2) if false.  (Other exit codes indicate an error querying the flag).  You can use it in shell scripts like this:

 if nbdinfo --is read-only nbd://localhost ||
    ! nbdinfo --can trim nbd://localhost
     error "the device must support writing and trimming"
nbdinfo --is read-only URI

Test if the server export is read-only.

nbdinfo --can write URI

For convenience this is the opposite of --is read-only.

nbdinfo --can read URI

All NBD servers must support read, so this always exits with success (unless there is a failure connecting to the URI).

nbdinfo --can connect URI

Test if we can connect to the NBD URI.

nbdinfo --is tls URI

Test if the NBD URI connection is using TLS.

nbdinfo --can structured-reply URI

Test if server can respond with structured replies (a prerequisite for supporting block status commands).

nbdinfo --is rotational URI

Test if the server export is backed by something which behaves like a rotating disk: accessing nearby blocks may be faster than random access and requests should be sorted to improve performance.  Many servers do not or cannot report this accurately.

nbdinfo --can cache URI
nbdinfo --can df URI
nbdinfo --can fast-zero URI
nbdinfo --can flush URI
nbdinfo --can fua URI
nbdinfo --can multi-conn URI
nbdinfo --can trim URI
nbdinfo --can zero URI

Test other properties of the NBD server export.


To show a map of which areas of the disk are allocated and sparse, use the --map option:

 $ nbdinfo --map nbd://localhost/
       0  1048576  0  data
 1048576  1048576  3  hole,zero

The fields are: start, size, type, description (optional).

The type field is an integer showing the raw value from the NBD protocol.  For some maps nbdinfo knows how to translate the type into a printable description.

To get parseable JSON output, add --json:

 $ nbdinfo --map --json nbd://localhost/
 [{ "offset": 0, "length": 1048576,
    "type": 0, "description": "data" },
  { "offset": 1048576, "length": 1048576,
    "type": 3, "description": "hole,zero" }]

By default this shows the "base:allocation" map, but you can show other maps too:

 $ nbdinfo --map=qemu:dirty-bitmap:bitmap nbd://localhost/
 0  1048576  1  dirty

For more information on NBD maps, see Metadata querying in the NBD protocol.

Map totals

Using --map --totals performs the same operation as --map but displays a summary of the total size of each type of allocation, in bytes and as a percentage (of the virtual size of the export).  This is useful for estimating how much real storage is used on the server, or might be required when copying a sparse image with nbdcopy(1).

In the example below, half (50.0%) of the disk is allocated data and half is unallocated:

 $ nbdinfo --map --totals nbd://localhost/
 1048576  50.0%  0  data
 1048576  50.0%  3  hole,zero

The fields are: total size in bytes, percentage of the virtual size, type, description (optional).

You can also get the same information in parseable form using --json:

 $ nbdinfo --map --totals --json nbd://localhost/
 [{ "size": 1048576, "percent": 50,
    "type": 0, "description": "data" },
  { "size": 1048576, "percent": 50,
    "type": 3, "description": "hole,zero" }]

As with the --map option, by default this shows the "base:allocation" map, but you can show the summary for other maps.

List all exports

To list all the exports available on an NBD server use the --list (-L) option.  To get parseable JSON output, add --json.

For example:

 $ nbdkit file dir=. --run 'nbdinfo --list "$uri"'
 protocol: newstyle-fixed without TLS
     export-size: 1931476992 (1842M)
     uri: nbd://localhost:10809/Fedora-Workstation-Live-x86_64-29-1.2.iso
     export-size: 3955556352 (3862848K)
     uri: nbd://localhost:10809/debian-10.4.0-amd64-DVD-1.iso


nbdinfo can also run an NBD server as a subprocess.  This requires an NBD server which understands systemd socket activation, such as qemu-nbd(8) or nbdkit(1).  All the usual nbdinfo modes can be used.

For example, to give general information or display the map of a qcow2 file:

 nbdinfo -- [ qemu-nbd -r -f qcow2 file.qcow2 ]
 nbdinfo --map -- [ qemu-nbd -r -f qcow2 file.qcow2 ]

Note that [ ... ] are separate parameters, and must be surrounded by spaces.  -- separates nbdinfo parameters from subprocess parameters.

Alternative tools

You could use qemu-img info (see qemu-img(1)) to query a single export from an NBD server.  qemu-nbd -L (see qemu-nbd(8)) can list NBD exports.  nbdsh(1) or the libnbd(3) API can be used for more complex queries.



Display brief command line help and exit.

--can cache
--can connect
--can df
--can fast-zero
--can flush
--can fua
--can multi-conn
--can read
--can structured-reply
--can trim
--can write
--can zero

Test properties of the NBD server export or the connection itself. The command does not print anything.  Instead it exits with success (exit code 0) if true, or failure (exit code 2) if false. (Other exit codes indicate an error querying the flag).

For further information see the NBD protocol and the following libnbd functions: nbd_can_cache(3), nbd_can_df(3), nbd_can_fast_zero(3), nbd_can_flush(3), nbd_can_fua(3), nbd_can_multi_conn(3), nbd_can_trim(3), nbd_can_zero(3), nbd_is_read_only(3), nbd_get_structured_replies_negotiated(3).


Enable or disable ANSI colours in output.  By default we use colours if the output seems to be a terminal, and disable them if not.


Mostly the information displayed comes from the metadata sent by the NBD server during the handshake.  However nbdinfo also downloads a small amount of data from the beginning of the export to try to probe the content with file(1).

When not using --list, the default is --content, ie.  probing the content.  To prevent content probing, use --no-content.

When using --list, the default is --no-content (since downloading from each export is expensive).  To enable content probing use --list --content.

--is read-only
--is rotational
--is tls

Test if the NBD server export is read-only and rotational, or whether the connection itself is using TLS.  The command does not print anything.  Instead it exits with success (exit code 0) if true, or failure (exit code 2) if false.  (Other exit codes indicate an error querying the flag).

For further information see the NBD protocol and the following libnbd functions: nbd_is_read_only(3), nbd_is_rotational(3), nbd_get_tls_negotiated(3).


The output is displayed in JSON format.


List all the exports on an NBD server.  The export name in the NBD URI is ignored.


Display the map (usually whether parts of the disk are allocated or sparse) of the given export.  This displays the "base:allocation" map by default, you can choose a different map with the optional parameter.

See the "Map" section above.

--map --totals
--map=MAP --totals

The same as --map, but displays a summary of the total size of each type of allocation.

See the "Map totals" section above.


Display only the size in bytes of the export.


Display the package name and version and exit.

See Also

libnbd(3), nbdcopy(1), nbddump(1), nbdfuse(1), nbdsh(1), nbdublk(1), file(1), jq(1), qemu-img(1), qemu-nbd(8).


Richard W.M. Jones

Eric Blake


This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

Referenced By

libnbd(3), libnbd-release-notes-1.10(1), libnbd-release-notes-1.16(1), libnbd-release-notes-1.4(1), libnbd-release-notes-1.6(1), libnbd-release-notes-1.8(1), nbdcopy(1), nbddump(1), nbdfuse(1), nbdkit(1), nbdkit-blocksize-policy-filter(1), nbdkit-release-notes-1.22(1), nbdkit-release-notes-1.24(1), nbdkit-tls(1), nbdsh(1), nbdublk(1).

2023-05-10 libnbd-1.16.1