ioctl_fideduperange(2) — Linux manual page


ioctl_fideduperange(2)     System Calls Manual    ioctl_fideduperange(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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);

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
       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         top

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

ERRORS         top

       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

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

VERSIONS         top

       Some filesystems may limit the amount of data that can be
       deduplicated in a single call.

STANDARDS         top


HISTORY         top

       Linux 4.5.

       It was previously known as BTRFS_IOC_FILE_EXTENT_SAME and was
       private to Btrfs.

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.

SEE ALSO         top


Linux man-pages (unreleased)     (date)           ioctl_fideduperange(2)

Pages that refer to this page: ioctl(2)