bionic (2) ioctl_fideduperange.2.gz

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 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
       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.

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 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.

       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 storage.

       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 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.

SEE ALSO

       ioctl(2)

COLOPHON

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