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By mistake, the qemu-storage-daemon nbd sample passes
"writable=on-storage-daemon" instead of "writable=on".

We'll update the readme, using the right qemu argument.

Signed-off-by: Lucian Petrut <>

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Windows Network Block Device (WNBD)

What is WNBD?

The WNBD project provides virtual block devices through a Storport Miniport driver. It can connect to a Network Block Device (NBD) server, which exposes device details and acts as an IO channel. As an alternative, it can dispatch IO commands to an userspace process using a DeviceIoControl based interface.

The project also provides the libwnbd.dll library, which handles the userspace and driver communication. It provides the following features:

  • Creating WNBD devices (optionally connecting to a NBD server)
  • Removing WNBD devices
  • Listing WNBD devices
  • Providing IO counters (driver as well as userspace counters)
  • Processing IO requests (when not using NBD)

WNBD provides a low level API (the *Ioctl* functions), as well as a high level API that includes the IO dispatching boilerplate. Please check the public headers for more details.


Windows 10.0.17763 (Windows Server 2019) is the minimum supported version. The WNBD driver will refuse to install on older unsupported versions.

Submitting patches

WNBD is licensed under LGPL v2.1.

Code contributions must include a valid "Signed-off-by" acknowledging the license for the modified or contributed file.

We do not require assignment of copyright to contribute code; code is contributed under the terms of the applicable license.

Please check SubmittingPatches.rst for more details.


How to build


Visual Studio 2019 build tools or GUI (Community version or above)

Windows Driver Kit 2104 (10.0.20348.0)

As an alternative, you may use a Docker Container that provides the build prerequisites.

wnbd-client uses Boost, other dependencies might be added as well in the future. The WNBD dependencies are fetched using this script, which is automatically invoked when building the solution.

Building and packaging

The following snippet builds the WNBD project and copies the generated binaries to $outDir. The "reinstall.ps1" script removes existing WNBD devices and drivers, proceeding to install the driver placed in the same directory.

git clone
msbuild wnbd\vstudio\wnbd.sln
# The following binaries can be archived and copied to a destination
# host. Use "reinstall.ps1" to install or reinstall the driver.
copy wnbd\vstudio\x64\Debug\driver\* $outDir
copy wnbd\vstudio\x64\Debug\wnbd-client.exe $outDir
copy wnbd\vstudio\x64\Debug\libwnbd.dll $outDir
copy wnbd\vstudio\reinstall.ps1 $outDir

This project allows building an MSI installer that bundles WNBD and the Ceph Windows clients.

How to install

MSI installer

The Ceph MSI installer bundles a signed version of the WNBD driver. It can be downloaded from here:

If however you're interested in the manual install procedure, check the following sections.


By default, the driver will be "test signed" as part of the build process. In order to install it, make sure that your target machine allows "test signed" drivers. To enable test signing mode on your target machine, please issue the following from an elevated command prompt:

bcdedit.exe /set testsigning yes

Please note that test signed drivers cannot be used when Secure Boot is enabled on the target machine. To check the Secure Boot configuration, issue Confirm-SecureBootUEFI from an elevated PowerShell prompt


A reboot is required after changing bcdedit settings

Those steps are not required when using a certified driver.

Install / Uninstall

You can use the wnbd-client.exe command line tool to install and remove the driver.

To install the driver, issue the following from an elevated PowerShell prompt:

.\wnbd-client.exe install-driver .\wnbd.inf

To uninstall the driver, issue the following from an elevated PowerShell prompt:

.\wnbd-client.exe uninstall-driver

The uninstall-driver command will hard disconnect any existing disk mappings and WNBD storage adapters and then remove any previous installations of the driver.

After installing the driver, you may want to copy wnbd-client.exe and libwnbd.dll to a directory that's part of the PATH environment variable.

Note that a host reboot may be required after installing or uninstalling the WNBD driver.


The following command can be used to retrieve the wnbd-client, libwnbd and wnbd driver version:

wnbd-client.exe -v
wnbd-client.exe: 0.2.1-3-g3dc976b
libwnbd.dll: 0.2.1-3-g3dc976b
wnbd.sys: 0.2.1-3-g3dc976b

The driver version may also be retrieved by right clicking the wnbd.sys file and going to the Details section of the Properties page.

Note that the driver version reported by the Windows Device Manager is actually the build timestamp included in the DriverVer section of the wnbd.inf file (e.g. DriverVer = 02/19/2021,

Ceph integration

Mapping and umapping RBD images is straightforward, just use rbd, part of the Ceph Windows port.

rbd device map $imageName
rbd device unmap $imageName

Mapping NBD devices

The following samples describe configuring a Linux NBD server and connecting to it using WNBD. Please check this page for more details about using NBD.

Use wnbd-client help [<command-name>] to get the full list of commands as well as the available options.

NBD server configuration

cat /etc/nbd-server/config
# If you want to run everything as root rather than the nbd user, you
# may either say "root" in the two following lines, or remove them
# altogether. Do not remove the [bgeneric] section, however.
port = 10809
user = nbd
group = nbd
includedir = /etc/nbd-server/conf.d

# What follows are export definitions. You may create as much of them as
# you want, but the section header has to be unique.
exportname = /image/path.img
port = 10809
copyonwrite = true

Mapping an NBD export

wnbd-client.exe map foo $nbdServerAddress --port 10809
Number Friendly Name            Serial Number   HealthStatus         OperationalStatus      Total Size Partition
------ -------------            -------------   ------------         -----------------      ---------- ----------
0      Msft Virtual Disk                        Healthy              Online                     127 GB GPT
1      WNBD WNBD_DISK           foo             Healthy              Online                     256 MB RAW

WARNING: when connecting to a local NBD server (e.g. qemu-nbd), make sure to disable caching on the NBD server side. Having Windows caching enabled on the WNBD disk side as well as the underlying NBD backend can lead to deadlocks when running on the same host. This issue does not affect Ceph.

For example, when using qemu-storage-daemon.exe locally, set like so:

    qemu-storage-daemon.exe `
      --blockdev driver=file,node-name=file,filename=e:\img\test_100gb.raw, `
      --blockdev driver=raw,node-name=raw,file=file `
      --nbd-server addr.type=inet,,addr.port=10809 `
      --export type=nbd,id=export,node-name=raw,name=test_100gb,writable=on

Listing mapped devices

wnbd-client.exe list
Pid         DiskNumber  Nbd    Owner            InstanceName
3508        1           true   wnbd-client      foo
4024        2           false  ceph-rbd-wnbd    rbd/rbd_win_10g

Unmapping the device

By default, a soft disconnect is attempted. If that fails or times out, a hard disconnect is performed as fallback.

A soft disconnect will notify other storage drivers that the disk is about to be removed. Such drivers can temporarily block the disk removal until ready (e.g. wait for pending IO, flush caches, wait for open files, etc).

Using the --debug flag will tell what's blocking the soft device removal (for example, a volume ID if there are open files).

A hard remove doesn't emit PnP notifications to other storage drivers, requesting the WNBD driver to remove the disk immediately.

wnbd-client.exe unmap test2
Number Friendly Name             Serial Number    HealthStatus         OperationalStatus      Total Size Partition
------ -------------             -------------    ------------         -----------------      ---------- ----------
0      Msft Virtual Disk                          Healthy              Online                     127 GB GPT

WNBD driver logging

To help develop and debug the driver we use the following facilities provided by the operating system:

Note that the log level as well as the logging facilities are configurable. Please check the driver options section for more details.

libwnbd can use a logger provided by the library consumer application, defaulting to stderr logging. So for example, if you're using libwnbd with Ceph, the according messages will go to the Ceph log. Driver logs will still be separate, using one or more of the logging facilities described in this section.


To view and collect the log messages via DbgPrint you can use either a debugger or, if you do not want to go through the process of attaching the debugger, you can simply use DebugView.


To collect messages using the WPP provider you can use the following snippet from an elevated command prompt:

logman create trace "WNBD_tracing_session" -p {E35EAF83-0F07-418A-907C-141CD200F252} 0xffffffff 0xff -o c:\TraceFile.etl -rt
logman start "WNBD_tracing_session"

When you want to stop collecting you can issue:

logman stop "WNBD_tracing_session"

To decode the generated ETL file you will need to have access to the debug symbols (wnbd.pdb) and use traceview utility from the WDK binary folder.


Error, warning and info ETW events will end up in the System section of the Windows Event Log.

Debug messages can be retrieved using tracing sessions, leveraging the tracelog and tracerpt tools from the WDK binary folder.

To start a trace session one can use:

tracelog -start WNBDEventdrv -guid #FFACC4E7-C115-4FE2-9D3C-80FAE73BAB91 -f WNBDEventdrv.etl

To stop:

tracelog -stop WNBDEventdrv

To display the trace use:

tracerpt WNBDEventdrv.etl

Driver options

Use the following command to retrieve the list of WNBD driver options:

wnbd-client.exe list-opt
LogLevel           : 1 (Default: 1)
NewMappingsAllowed : true (Default: true)
EtwLoggingEnabled  : true (Default: true)
WppLoggingEnabled  : false (Default: false)
DbgPrintEnabled    : true (Default: true)
MaxIOReqPerAdapter : 1000 (Default: 1000)
MaxIOReqPerLun     : 255 (Default: 255)
RemoveStaleConnections : true (Default: true)
StaleReqTimeoutMs      : 15000 (Default: 15000)
StaleConnTimeoutMs     : 60000 (Default: 60000)

Use the following command to configure an option. If the setting should persist across reboots, pass the optional --persistent argument.

The log level matches the DebugPrint filter levels:

wnbd-client.exe set-opt LogLevel 4 --persistent

To list only the persistent options, pass the --persistent parameter:

wnbd-client.exe list-opt --persistent
LogLevel           : 4 (Default: 1)

To clear a custom setting and reapply the default value, use the following command:

wnbd-client.exe reset-opt LogLevel

Passing the --persistent flag will remove the persistent setting as well.

Adjusting IO queue size

By default, Storport allows up to 1000 concurrent IO requests per adapter and 255 requests per disk. This may be insufficient for large workloads, which is why the WNBD IO limits can be adjusted using the following settings:

  • MaxIOReqPerAdapter - min: 1, max: 131072
  • MaxIOReqPerLun - min: 1, max: 1024

The WNBD adapter must be reset in order for the new IO limits to be applied.

wnbd-client.exe set-opt MaxIOReqPerAdapter 8192 --persistent
wnbd-client.exe set-opt MaxIOReqPerLun 512 --persistent

# When using Ceph, stop the "ceph-rbd" service so that the existing
# mappings will be gracefully disconnected.
stop-service ceph-rbd

wnbd-client.exe reset-adapter --hard-disconnect-mappings

start-service ceph-rbd

# Verify the updated IO limits
wnbd-client.exe show $mapping

Stale IO daemon detection

In some situations, the host can become unresponsive if there are stale IO connections.

To avoid this, WNBD automatically detects and removes stale connections. This behavior along with the timeouts are configurable using the following settings:

RemoveStaleConnections : true (Default: true)
StaleReqTimeoutMs      : 15000 (Default: 15000)
StaleConnTimeoutMs     : 60000 (Default: 60000)

The driver considers the connection to be stale if there are aborted requests older than StaleReqTimeoutMs and if there hasn't been any IO reply since StaleConnTimeoutMs.


Hyper-V disk addressing

Warning: Hyper-V identifies passthrough VM disks by number instead of SCSI ID, although the disk number can change across host reboots. This means that the VMs can end up using incorrect disks after rebooting the host, which is an important security concern. This issue also affects iSCSI and Fibre Channel disks.

There are a few possible ways of avoiding this Hyper-V limitation:

  • use an NTFS/ReFS partition to store VHDX image files instead of directly attaching the WNBD disk. This may slightly impact the IO performance.
  • use the Hyper-V AutomaticStartAction setting to prevent the VMs from booting with the incorrect disks and have a script that updates VM disks attachments before powering them back on. The ElementName field of the Msvm_StorageAllocationSettingData WMI class may be used to label VM disk attachments.
  • use the Openstack Hyper-V driver, which automatically refreshes the VM disk attachments before powering them back on.



Most wnbd-client commands require privileged rights. Make sure to use an elevated PowerShell or CMD command prompt. That also applies when using other clients that leverage the libwnbd library, such as Ceph clients.

Crash dumps

Windows stores kernel crash dumps at %SystemRoot%\Memory.dmp, which is tipically C:\Windows\Memory.dmp.

Userspace crash dumps can be placed at a configurable location and enabled for all applications or just predefined ones, as outlined here.

Whenever a Windows application crashes, an event will be submitted to the Application Windows Event Log, having Event ID 1000. The entry will also include the process id, the faulting module name and path as well as the exception code.

Please note that in order to analyze crash dumps, the debug symbols (.pdb files) are required.


Please see the logging and driver options sections.

You can start by checking the System Windows Event Log for wnbd driver errors. If needed, increase the wnbd log level and start a tracing session.

IO counters

The following command can be used to retrieve per-disk IO counters that are gathered at the driver level:

wnbd-client.exe stats $mapping
Disk stats
TotalReceivedIORequests        : 70
TotalSubmittedIORequests       : 0
TotalReceivedIOReplies         : 70
UnsubmittedIORequests          : 0
PendingSubmittedIORequests     : 0
AbortedSubmittedIORequests     : 0
AbortedUnsubmittedIORequests   : 0
CompletedAbortedIORequests     : 0
OutstandingIOCount             : 0
TimeSinceLastReceivedReqMs     : 3037
TimeSinceLastSubmittedReqMs    : 3037
TimeSinceLastReplyMs           : 3037

The received requests are the ones coming from Storport, the driver upper layer. The submitted requests are IO requests that have been forwarded to the storage backend (e.g. NBD server). The replies counter shows replies coming from the storage backend. Aborted requests are usually requests that have timed out, typically after 10 up to 30 seconds.

Those counters are very useful when debugging stuck IO, helping us understand the state of the requests. The userspace libwnbd library also gathers IO counters, which can be retrieved by the library consumers.

Worth mentioning that by default, Storport limits the number of pending IO requests to 1000 per adapter and 255 per LUN. Those limits can be adjusted as described above.

Be aware that the IO timers such as TimeSinceLastReceivedReqMs have a ~15ms accuracy and are mostly intended for timeout detection.