ioctl_fideduperange(2) — Linux manual page



NAME         top

       ioctl_fideduperange  -  share  some the data of one file with another

SYNOPSIS         top

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

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

DESCRIPTION         top

       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 ar‐
       ray 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 de‐
       scriptor 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 en‐
       forced 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 suc‐
       cessfully 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         top

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

ERRORS         top

       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
              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

       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

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

VERSIONS         top

       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         top

       This API is Linux-specific.

NOTES         top

       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         top


COLOPHON         top

       This page is part of release 5.09 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)

Pages that refer to this page: ioctl(2)