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WinBtrfs v1.0.2

WinBtrfs is a Windows driver for the next-generation Linux filesystem Btrfs. A reimplementation from scratch, it contains no code from the Linux kernel, and should work on any version from Windows 7 onwards. It is also included as part of the free operating system ReactOS.

First, a disclaimer:

This software is in active development - YOU USE IT AT YOUR OWN RISK. I take NO RESPONSIBILITY for any damage it may do to your filesystem. DO NOT USE THIS DRIVER UNLESS YOU HAVE FULL AND UP-TO-DATE BACKUPS OF ALL YOUR DATA. Do not rely on Btrfs' internal mechanisms: SNAPSHOTS ARE NOT BACKUPS, AND DO NOT RULE OUT THE POSSIBILITY OF SILENT CORRUPTION.

In other words, assume that the driver is going to corrupt your entire filesystem, and you'll be pleasantly surprised when it doesn't.

However, having said that, it ought to be suitable for day-to-day use.

Everything here is released under the GNU Lesser General Public Licence (LGPL); see the file LICENCE for more info. You are encouraged to play about with the source code as you will, and I'd appreciate a note ( if you come up with anything nifty. On top of that, I'm open to relicensing the code if you've a burning desire to use it on a GPL or commercial project, or what have you - drop me a line and we'll talk.

See at the end of this document for copyright details of third-party code that's included here.


I've been developing this driver for fun, and in the hopes that someone out there will find it useful. But if you want to provide some pecuniary encouragement, it'd be very much appreciated:


  • Reading and writing of Btrfs filesystems
  • Basic RAID: RAID0, RAID1, and RAID10
  • Advanced RAID: RAID5 and RAID6 (incompat flag raid56)
  • Caching
  • Discovery of Btrfs partitions, even if Windows would normally ignore them
  • Getting and setting of Access Control Lists (ACLs), using the xattr security.NTACL
  • Alternate Data Streams (e.g. :Zone.Identifier is stored as the xattr user.Zone.Identifier)
  • Supported incompat flags: mixed_backref, default_subvol, big_metadata, extended_iref, skinny_metadata.
  • Mappings from Linux users to Windows ones (see below)
  • Symlinks and other reparse points
  • Shell extension to identify and create subvolumes, including snapshots
  • Hard links
  • Sparse files
  • Free-space cache
  • Preallocation
  • Asynchronous reading and writing
  • Partition-less Btrfs volumes
  • Per-volume registry mount options (see below)
  • zlib compression
  • LZO compression (incompat flag compress_lzo)
  • Misc incompat flags: mixed_groups, no_holes
  • LXSS ("Ubuntu on Windows") support
  • Balancing (including resuming balances started on Linux)
  • Device addition and removal
  • Creation of new filesystems with mkbtrfs.exe and ubtrfs.dll
  • Scrubbing
  • Reflink copy
  • Subvol send and receive
  • Degraded mounts
  • Free space tree (compat_ro flag free_space_cache)
  • Shrinking and expanding


  • Passthrough of permissions etc. for LXSS
  • Oplocks


To install the driver, download and extract the latest release, right-click btrfs.inf, and choose Install. The driver is signed, so should work out of the box on modern versions of Windows.

For the very latest versions of Windows 10, Microsoft introduced more onerous requirements for signing, which are only available to corporations and not individuals. If this affects you (i.e. you get a signing error when trying to install the driver), try disabling Secure Boot in your BIOS settings.


If you want to uninstall, go to Device Manager, find "Btrfs controller" under "Storage volumes", right click and choose "Uninstall". Tick the checkbox to uninstall the driver as well, and let Windows reboot itself.

If you need to uninstall via the registry, open regedit and set the value of HKLM\SYSTEM\CurrentControlSet\services\btrfs\Start to 4, to disable the service. After you reboot, you can then delete the btrfs key and remove C:\Windows\System32\drivers\btrfs.sys.


You will need Microsoft Visual C++ 2015 if you want to compile the driver; you might be able to get earlier versions to work with a bit of work.

You'll also need a copy of the Windows DDK; I placed mine in C:\WinDDK. If yours is somewhere else, you'll need to edit the project settings. You'll also need to edit the post-build steps for the 64-bit versions, which are set up to self-sign using my own certificate.


The user mappings are stored in the registry key HKLM\SYSTEM\CurrentControlSet\services\btrfs\Mappings. Create a DWORD with the name of your Windows SID (e.g. S-1-5-21-1379886684-2432464051-424789967-1001), and the value of your Linux uid (e.g. 1000). It will take effect next time the driver is loaded.

You can find your current SID by running wmic useraccount get name,sid.

Similarly, the group mappings are stored in under GroupMappings. The default entry maps Windows' Users group to gid 100, which is usually "users" on Linux. You can also specify user SIDs here to force files created by a user to belong to a certain group. The setgid flag also works as on Linux.


The DLL file shellbtrfs.dll provides the GUI interface, but it can also be used with rundll32.exe to carry out some tasks from the command line, which may be useful if you wish to schedule something to run periodically.

Bear in mind that rundll32 provides no mechanism to return any error codes, so any of these commands may fail silently.

  • rundll32.exe shellbtrfs.dll,CreateSubvol <path>

  • rundll32.exe shellbtrfs.dll,CreateSnapshot <source> <destination>

  • rundll32.exe shellbtrfs.dll,ReflinkCopy <source> <destination> This also accepts wildcards, and any number of source files.

The following commands need various privileges, and so must be run as Administrator to work:

  • rundll32.exe shellbtrfs.dll,SendSubvol <source> [-p <parent>] [-c <clone subvol>] <stream file> The -p and -c flags are as btrfs send on Linux. You can specify any number of clone subvolumes.

  • rundll32.exe shellbtrfs.dll,RecvSubvol <stream file> <destination>

  • rundll32.exe shellbtrfs.dll,StartScrub <drive>

  • rundll32.exe shellbtrfs.dll,StopScrub <drive>


  • The filenames are weird! or
  • I get strange errors on certain files or directories!

The driver assumes that all filenames are encoded in UTF-8. This should be the default on most setups nowadays - if you're not using UTF-8, it's probably worth looking into converting your files.

  • btrfs check reports errors in the extent tree

There's a bug in btrfs-progs v4.7, which causes it to return false positives when using prealloc extents - this'll also manifest itself with filesystems from the official driver. If you still get the same errors when using btrfs-check v4.6, please e-mail me what it says.

  • The root of the drive isn't case-sensitive in LXSS

This is something Microsoft hardcoded into LXSS, presumably to stop people hosing their systems by running mkdir /mnt/c/WiNdOwS.

  • Disk Management doesn't work properly, e.g. unable to change drive letter

Try changing the type of your partition in Linux. For MBR partitions, this should be type 7 in fdisk. For GPT partitions, this should be type 6 in fdisk ("Microsoft basic data"), or 0700 in gdisk. We have to do some chicanery to get Linux partitions to appear in the first place, but unfortunately this confuses diskmgmt.msc too much.

  • How do I format a partition as Btrfs?

Use the included command line program mkbtrfs.exe. We can't add Btrfs to Windows' own dialog box, unfortunately, as its list of filesystems has been hardcoded. You can also run format /fs:btrfs, if you don't need to set any Btrfs-specific options.

  • I can't reformat a mounted Btrfs filesystem

If Windows' Format dialog box refuses to appear, try running with the /fs flag, e.g. format /fs:ntfs D:.


v1.0.2 (2018-05-19):

  • Minor bug fixes

v1.0.1 (2017-10-15):

  • Fixed deadlock
  • Binaries now signed
  • Minor bug fixes

v1.0 (2017-09-04):

  • First non-beta release!
  • Degraded mounts
  • New free space cache (compat_ro flag free_space_cache)
  • Shrinking and expanding of volumes
  • Registry options now re-read when changed, rather than just on startup
  • Improved balancing on very full filesystems
  • Fixed problem preventing user profile directory being stored on btrfs on Windows 8 and above
  • Better Plug and Play support
  • Miscellaneous bug fixes

v0.10 (2017-05-02):

  • Reflink copy
  • Sending and receiving subvolumes
  • Group mappings (see Mappings section above)
  • Added commands for scripting etc. (see Commands section above)
  • Fixed an issue preventing mounting on non-PNP devices, such as VeraCrypt
  • Fixed an issue preventing new versions of LXSS from working
  • Fixed problem with the ordering of extent refs, which caused problems on Linux but wasn't picked up by btrfs check
  • Added support for reading compressed inline extents
  • Many miscellaneous bug fixes

v0.9 (2017-03-05):

  • Scrubbing
  • Better handling of multi-device volumes
  • Performance increases when reading from RAID filesystems
  • No longer lies about being NTFS, except when it has to
  • Volumes will now go readonly if there is an unrecoverable error, rather than blue-screening
  • Filesystems can now be created with Windows' inbuilt
  • Zlib upgraded to version 1.2.11
  • Miscellaneous performance increases
  • Miscellaneous bug fixes

v0.8 (2016-12-30):

  • Volume property sheet, for:
  • Balances
  • Adding and removing devices
  • Showing disk usage, i.e. the equivalent to btrfs fi usage
  • Checksums now calulated in parallel where appropriate
  • Creation of new filesystems, with mkbtrfs.exe
  • Plug and play support for RAID devices
  • Disk usage now correctly allocated to processes in taskmgr
  • Performance increases
  • Miscellaneous bug fixes

v0.7 (2016-10-24):

  • Support for RAID5/6 (incompat flag raid56)
  • Seeding support
  • LXSS ("Ubuntu on Windows") support
  • Support for Windows Extended Attributes
  • Improved removable device support
  • Better snapshot support
  • Recovery from RAID checksum errors
  • Fixed issue where creating a lot of new files was taking a long time
  • Miscellaneous speed increases and bug fixes

v0.6 (2016-08-21):

  • Compression support (both zlib and lzo)
  • Mixed groups support
  • No-holes support
  • Added inode property sheet to shell extension
  • Many more mount options (see below)
  • Better support for removable devices
  • Page file support
  • Many miscellaneous bug fixes

v0.5 (2016-07-24):

  • Massive speed increases (from "sluggish" to "blistering")
  • Massive stability improvements
  • RAID support: RAID0, RAID1, and RAID10
  • Asynchronous reading and writing
  • Partition-less Btrfs volumes
  • Windows sparse file support
  • Object ID support
  • Beginnings of per-volume mount options
  • Security improvements
  • Notification improvements
  • Miscellaneous bug fixes

v0.4 (2016-05-02):

  • Subvolume creation and deletion
  • Snapshots
  • Preallocation
  • Reparse points
  • Hard links
  • Plug and play
  • Free-space cache
  • Fix problems preventing volume from being shared over the network
  • Miscellaneous bug fixes

v0.3 (2016-03-25):

  • Bug fixes:
  • Fixed crashes when metadata blocks were SINGLE, such as on SSDs
  • Fixed crash when splitting an internal tree
  • Fixed tree traversal failing when first item in tree had been deleted
  • Fixed emptying out of whole tree (probably only relevant to checksum tree)
  • Fixed "incorrect local backref count" message appearing in btrfs check
  • Miscellaneous other fixes
  • Added beginnings of shell extension, which currently only changes the icon of subvolumes

v0.2 (2016-03-13):

  • Bug fix release:
  • Check memory allocations succeed
  • Check tree items are the size we're expecting
  • Added rollbacks, so failed operations are completely undone
  • Fixed driver claiming all unrecognized partitions (thanks Pierre Schweitzer)
  • Fixed deadlock within CcCopyRead
  • Fixed changing properties of a JPEG within Explorer
  • Lie about FS type, so UAC works
  • Many, many miscellaneous bug fixes
  • Rudimentary security support
  • Debug log support (see below)

v0.1 (2016-02-21):

  • Initial alpha release.

Debug log

WinBtrfs has three levels of debug messages: errors and FIXMEs, warnings, and traces. The release version of the driver only displays the errors and FIXMEs, which it logs via DbgPrint. You can view these messages via the Microsoft program DebugView, available at

If you want to report a problem, it'd be of great help if you could also attach a full debug log. To do this, you will need to use the debug versions of the drivers; copy the files in Debug\x64 or Debug\x86 into x64 or x86. You will also need to set the registry entries in HKLM\SYSTEM\CurrentControlSet\Services\btrfs:

  • DebugLogLevel (DWORD): 0 for no messages, 1 for errors and FIXMEs, 2 for warnings also, and 3 for absolutely everything, including traces.
  • LogDevice (string, optional): the serial device you want to output to, such as \Device\Serial0. This is probably only useful on virtual machines.
  • LogFile (string, optional): the file you wish to output to, if LogDevice isn't set. Bear in mind this is a kernel filename, so you'll have to prefix it with "\??\" (e.g., "\??\C:\btrfs.log"). It probably goes without saying, but don't store this on a volume the driver itself is using, or you'll cause an infinite loop.

Mount options

The driver will create subkeys in the registry under HKLM\SYSTEM\CurrentControlSet\Services\btrfs for each mounted filesystem, named after its UUID. If you're unsure which UUID refers to which volume, you can check using btrfs fi show on Linux. You can add per-volume mount options to this subkey, which will take effect on reboot. If a value is set in the key above this, it will use this by default.

  • Ignore (DWORD): set this to 1 to tell the driver not to attempt loading this filesystem. With the `Readonly flag, this is probably redundant.

  • Readonly (DWORD): set this to 1 to tell the driver not to allow writing to this volume. This is the equivalent of the ro flag on Linux.

  • Compress (DWORD): set this to 1 to tell the driver to write files as compressed by default. This is the equivalent of the compress flag on Linux.

  • CompressForce (DWORD): set this to 1 to force compression, i.e. to ignore the nocompress inode flag and even attempt compression of incompressible files. This isn't a good idea, but is the equivalent of the compress-force flag on Linux.

  • CompressType (DWORD): set this to 1 to prefer zlib compression, and 2 to prefer lzo compression. The default is 0, which uses lzo compression if the incompat flag is set, and zlib otherwise.

  • FlushInterval (DWORD): the interval in seconds between metadata flushes. The default is 30, as on Linux - the parameter is called commit there.

  • ZlibLevel (DWORD): a number between -1 and 9, which determines how much CPU time is spent trying to compress files. You might want to fiddle with this if you have a fast CPU but a slow disk, or vice versa. The default is 3, which is the hard-coded value on Linux.

  • MaxInline (DWORD): the maximum size that will be allowed for "inline" files, i.e. those stored in the metadata. The default is 2048, which is also the default on modern versions of Linux - the parameter is called max_inline there. It will be clipped to the maximum value, which unless you've changed your node size will be a shade under 16 KB.

  • SubvolId (QWORD): the ID of the subvolume that we will attempt to mount as the root. If it doesn't exist, this parameter will be silently ignored. The subvolume ID can be found on the inode property sheet; it's in hex there, as opposed to decimal on the Linux tools. The default is whatever has been set via btrfs subvolume set-default; or, failing that, subvolume 5. The equivalent parameter on Linux is called subvolid.

  • SkipBalance (DWORD): set to 1 to tell the driver not to attempt resuming a balance which was running when the system last powered down. The default is 0. The equivalent parameter on Linux is skip_balance.

  • NoPNP (DWORD): useful for debugging only, this forces any volumes to appear rather than exposing them via the usual Plug and Play method.


I'd appreciate any feedback you might have, positive or negative:


This code also contains portions of zlib, which is licensed as follows:

Copyright (C) 1995-2017 Jean-loup Gailly and Mark Adler

This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.

It also contains portions of an early version of lzo, which is copyright 1996 Markus Oberhumer. Modern versions are licensed under the GPL, but this was licensed under the LGPL, so I believe it is okay to use.