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

NAME

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

SYNOPSIS

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

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

DESCRIPTION

     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
     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  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
     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.

RETURN VALUE

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

ERRORS

     Error codes can be one of, but are not limited to, the following:

     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.

     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.

     EOPNOTSUPP
            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.

     ETXTBSY
            One of the files is a swap file.  Swap files cannot share  stor-
            age.

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

VERSIONS

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

CONFORMING TO

     This API is Linux-specific.

NOTES

     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.