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From: Mark Fasheh <mfasheh <at> suse.de>
Subject: [PATCH 0/4] btrfs: offline dedupe v2
Newsgroups: gmane.comp.file-systems.btrfs
Date: Tuesday 11th June 2013 20:31:34 UTC (over 5 years ago)

The following series of patches implements in btrfs an ioctl to do
offline deduplication of file extents.

To be clear, "offline" in this sense means that the file system is
mounted and running, but the dedupe is not done during file writes,
but after the fact when some userspace software initiates a dedupe.

The primary patch is loosely based off of one sent by Josef Bacik back
in January, 2011.


I've made significant updates and changes from the original. In
particular the structure passed is more fleshed out, this series has a
high degree of code sharing between itself and the clone code, and the
locking has been updated.

The ioctl accepts a struct:

struct btrfs_ioctl_same_args {
	__u64 logical_offset;	/* in - start of extent in source */
	__u64 length;		/* in - length of extent */
	__u16 dest_count;	/* in - total elements in info array */
	__u16 reserved1;
	__u32 reserved2;
	struct btrfs_ioctl_same_extent_info info[0];

Userspace puts each duplicate extent (other than the source) in an
item in the info array. As there can be multiple dedupes in one
operation, each info item has it's own status and 'bytes_deduped'
member. This provides a number of benefits:

- We don't have to fail the entire ioctl because one of the dedupes failed.

- Userspace will always know how much progress was made on a file as we
  return the number of bytes deduped.

/* For extent-same ioctl */
struct btrfs_ioctl_same_extent_info {
	__s64 fd;		/* in - destination file */
	__u64 logical_offset;	/* in - start of extent in destination */
	__u64 bytes_deduped;	/* out - total # of bytes we were able
				 * to dedupe from this file */
	/* status of this dedupe operation:
	 * 0 if dedup succeeds
	 * < 0 for error
	 * == BTRFS_SAME_DATA_DIFFERS if data differs
	__s32 status;		/* out - see above description */
	__u32 reserved;

The kernel patches are based off Linux v3.9. At this point I've tested the
ioctl against a decent variety of files and conditions.

A git tree for the kernel changes can be found at:


I have a userspace project, duperemove available at:


Hopefully this can serve as an example of one possible usage of the ioctl.

duperemove takes a list of files as argument and will search them for
duplicated extents. If given the '-D' switch, duperemove will send dedupe
requests for same extents and display the results.

Within the duperemove repo is a file, btrfs-extent-same.c that acts as
a test wrapper around the ioctl. It can be compiled completely
seperately from the rest of the project via "make
btrfs-extent-same". This makes direct testing of the ioctl more


We can't yet dedupe within the same file (that is, source and destination
are the same inode). This is due to a limitation in btrfs_clone().

Perhaps this isn't a limiation per-se but extent-same requires read/write
access to the files we want to dedupe.  During my last series I had a
conversation with Gabriel de Perthuis about access checking where we tried
to maintain the ability for a user to run extent-same against a readonly
snapshot. In addition, I reasoned that since the underlying data won't
change (at least to the user) that we ought only require the files to be
open for read.

What I found however is that neither of these is a great idea ;)

- We want to require that the inode be open for writing so that an
  unprivileged user can't do things like run dedupe on a performance
  sensitive file that they might only have read access to.  In addition I
  could see it as kind of a surprise (non-standard behavior) to an
  administrator that users could alter the layout of files they are only
  allowed to read.

- Readonly snapshots won't let you open for write anyway (unsuprisingly,
  open() returns -EROFS).  So that kind of kills the idea of them being
  to open those files for write which we want to dedupe.

That said, I still think being able to run this against a set of readonly
snapshots makes sense especially if those snapshots are taken for backup
purposes. I'm just not sure how we can sanely enable it.

Code review is very much appreciated. Thanks,


- check that we have appropriate access to each file before deduping. For
  the source, we only check that it is opened for read. Target files have
  be open for write.

- don't dedupe on readonly submounts (this is to maintain 

- check that we don't dedupe files with different checksumming states
 (compare BTRFS_INODE_NODATASUM flags)

- get and maintain write access to the mount during the extent same
  operation (mount_want_write())

- allocate our read buffers up front in btrfs_ioctl_file_extent_same() and
  pass them through for re-use on every call to btrfs_extent_same().
  to David Sterba  for reporting this

- As the read buffers could possibly be up to 1MB (depending on user
  request), we now conditionally vmalloc them.

- removed redundant check for same inode. btrfs_extent_same() catches it
  and bubbles the error up.

- remove some unnecessary printks

Changes from RFC to v1:

- don't error on large length value in btrfs exent-same, instead we just
  dedupe the maximum allowed.  That way userspace doesn't have to worry
  about an arbitrary length limit.

- btrfs_extent_same will now loop over the dedupe range at 1MB increments
  a total of 16MB per request)

- cleaned up poorly coded while loop in __extent_read_full_page() (thanks
  David Sterba  for reporting this)

- included two fixes from Gabriel de Perthuis :
   - allow dedupe across subvolumes
   - don't lock compressed pages twice when deduplicating

- removed some unused / poorly designed fields in btrfs_ioctl_same_args.
  This should also give us a bit more reserved bytes.

- return -E2BIG instead of -ENOMEM when arg list is too large (thanks to
  David Sterba  for reporting this)

- Some more reserved bytes are now included as a result of some of my
  cleanups. Quite possibly we could add a couple more.
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