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


       Standard C library (libc, -lc)


       #include <linux/fs.h>      /* Definition of FIDEDUPERANGE and
                                     FILE_DEDUPE_* constants*/
       #include <sys/ioctl.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 consumption 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 deduplicated 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  destinations  is  given  in  dest_count,  and  the  destination
       information is conveyed in the following form:

           struct file_dedupe_range_info {
               __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  combined  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 dependent 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

       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.


       Possible errors include (but are not limited to) the following:

       EBADF  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

       EINVAL 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.

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

       ENOMEM 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.

       EPERM  dest_fd is immutable.

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

       EXDEV  dest_fd and src_fd are not on the same mounted filesystem.


       This   ioctl  operation  first  appeared  in  Linux  4.5.   It  was  previously  known  as
       BTRFS_IOC_FILE_EXTENT_SAME and was private 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.