ioctl_fideduperange - share some the data of one file with
          another file

          #include <sys/ioctl.h>
          #include <linux/fs.h>

          int ioctl(int src_fd, FIDEDUPERANGE, struct
          file_dedupe_range *arg);

          If a filesystem supports files sharing physical storage
          between multiple files, this ioctl(2) operation can be used
          to make some of the data in the src_fd file appear in the
          dest_fd file by sharing the underlying storage if the file
          data is identical ("deduplication").  Both files must reside
          within the same filesystem.  This reduces storage consump-
          tion by allowing the filesystem to store one shared copy of
          the data.  If a file write should occur to a shared region,
          the filesystem must ensure that the changes remain private
          to the file being written.  This behavior is commonly
          referred to as "copy on write".

          This ioctl performs the "compare and share if identical"
          operation on up to src_length bytes from file descriptor
          src_fd at offset src_offset. This information is conveyed in
          a structure of the following form:

              struct file_dedupe_range {
                  __u64 src_offset;
                  __u64 src_length;
                  __u16 dest_count;
                  __u16 reserved1;
                  __u32 reserved2;
                  struct file_dedupe_range_info info[0];

          Deduplication is atomic with regards to concurrent writes,
          so no locks need to be taken to obtain a consistent dedupli-
          cated copy.

          The fields reserved1 and reserved2 must be zero.

          Destinations for the deduplication operation are conveyed in
          the array at the end of the structure.  The number of desti-
          nations is given in dest_count, and the destination informa-
          tion is conveyed in the following form:

              struct file_dedupe_range_info {

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                  __s64 dest_fd;
                  __u64 dest_offset;
                  __u64 bytes_deduped;
                  __s32 status;
                  __u32 reserved;

          Each deduplication operation targets src_length bytes in
          file descriptor dest_fd at offset dest_offset. The field
          reserved must be zero.  During the call, src_fd must be open
          for reading and dest_fd must be open for writing.  The com-
          bined size of the struct file_dedupe_range and the struct
          file_dedupe_range_info array must not exceed the system page
          size.  The maximum size of src_length is filesystem depen-
          dent and is typically 16 MiB.  This limit will be enforced
          silently by the filesystem.  By convention, the storage used
          by src_fd is mapped into dest_fd and the previous contents
          in dest_fd are freed.

          Upon successful completion of this ioctl, the number of
          bytes successfully deduplicated is returned in bytes_deduped
          and a status code for the deduplication operation is
          returned in status. If even a single byte in the range does
          not match, the deduplication request will be ignored and
          status set to FILE_DEDUPE_RANGE_DIFFERS.  The status code is
          set to FILE_DEDUPE_RANGE_SAME for success, a negative error
          code in case of error, or FILE_DEDUPE_RANGE_DIFFERS if the
          data did not match.

          On error, -1 is returned, and errno is set to indicate the

          Error codes can be one of, but are not limited to, the fol-

               src_fd is not open for reading; dest_fd is not open for
               writing or is open for append-only writes; or the
               filesystem which src_fd resides on does not support

               The filesystem does not support deduplicating the
               ranges of the given files.  This error can also appear
               if either file descriptor represents a device, FIFO, or
               socket.  Disk filesystems generally require the offset
               and length arguments to be aligned to the fundamental
               block size.  Neither Btrfs nor XFS support overlapping
               deduplication ranges in the same file.

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               One of the files is a directory and the filesystem does
               not support shared regions in directories.

               The kernel was unable to allocate sufficient memory to
               perform the operation or dest_count is so large that
               the input argument description spans more than a single
               page of memory.

               This can appear if the filesystem does not support
               deduplicating either file descriptor, or if either file
               descriptor refers to special inodes.

               dest_fd is immutable.

               One of the files is a swap file.  Swap files cannot
               share storage.

               dest_fd and src_fd are not on the same mounted filesys-

          This ioctl operation first appeared in Linux 4.5.  It was
          previously known as BTRFS_IOC_FILE_EXTENT_SAME and was pri-
          vate to Btrfs.

          This API is Linux-specific.

          Because a copy-on-write operation requires the allocation of
          new storage, the fallocate(2) operation may unshare shared
          blocks to guarantee that subsequent writes will not fail
          because of lack of disk space.

          Some filesystems may limit the amount of data that can be
          deduplicated in a single call.


          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at

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