WinBtrfs is a Windows driver for the next-generation Linux filesystem Btrfs. The ultimate aim is for it to be feature-complete, but most of the basics are there already. It is a reimplementation from scratch, and contains no code from the Linux kernel. 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.

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 (mark@harmstone.com) 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.

• Reading and writing of Btrfs filesystems
• 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
• Free-space cache
• Preallocation

• Basic RAID: RAID0, RAID1, and RAID10
• RAID5 and RAID6 (incompat flag raid56)
• zlib compression
• LZO compression (incompat flag compress_lzo)
• New (Linux 4.5) free space cache (compat_ro flag free_space_cache)
• Misc incompat flags: mixed_groups, no_holes
• Asynchronous reading and writing
• Probably a bunch of other bugs

The driver is self-signed at the moment, meaning that if you're using a 64-bit version of Windows you'll have to tell it to boot up in Test Mode if you want it to work. To do this, launch an admin command prompt (right-click on "Command Prompt" and click "Run as administrator"), and run the following command:

bcdedit -set TESTSIGNING ON

Reboot, and you should see "Test Mode" on the bottom right of the Desktop.

If you just want to test the driver out, run loader.exe as an Administrator. Despite what it says, you can close it - the driver will stay in memory until you shutdown. Be warned that currently running programs, including the Desktop, might not display your Btrfs partition until they're restarted - your drive might not appear in My Computer, but if you run E:\ (for example), it'll show up.

If you're feeling adventurous and want to install the driver permanently, right-click btrfs.inf, click Install, and reboot.

You will need Microsoft Visual C++ if you want to compile the driver; I used the 2008 edition, but later versions should work too. I've not been able to get it to work with GCC; it worked for a while, then suddenly stopped when the code got to a certain size. If you've got any clues about what this is all about, I'd appreciate it if you sent me an e-mail.

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.

• My drive doesn't show up!

Check:

a) If on 64-bit Windows, you're running in Test Mode ("Test Mode" appears in the bottom right of the Desktop).

b) You're not trying to mount a multi-device filesystem, which isn't supported yet.

c) You're not trying to mount a filesystem with unsupported flags. On Linux, type:

ls /sys/fs/btrfs/*/features/

If you see any of the flags listed above as being unsupported, it won't work.

• I can't read from or write to a particular file!

Make sure it's not compressed, which isn't yet supported.

• 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.

• Windows thinks it's an NTFS volume

Unfortunately we have to lie about this - the function MPR!MprGetConnection checks the filesystem type against an internal whitelist, and fails if it's not found, which prevents UAC from working. Thanks Microsoft!

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.

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 https://technet.microsoft.com/en-gb/sysinternals/debugview.

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.

I'd appreciate any feedback you might have, positive or negative: mark@harmstone.com.