IOCTL-FIDEDUPERANGE(2)     Linux Programmer's Manual    IOCTL-FIDEDUPERANGE(2)

       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 multi-
       ple 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_off-
       set.   This  information  is  conveyed  in a structure of the following

           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 follow-
       ing 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  descrip-
       tor  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 freed.

       Upon successful completion of this ioctl, the number of bytes  success-
       fully  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 ig-
       nored 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.

       Error codes can be one of, 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 deduplication.

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

       EISDIR One of the files is a directory and the filesystem does not sup-
              port 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  stor-

       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  stor-
       age,  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 5.05 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

Linux                             2019-10-10            IOCTL-FIDEDUPERANGE(2)
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