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 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 length bytes in file descriptor dest_fd at offset logical_offset. The field reserved must be zero. 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. The status code is set to 0 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:

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.

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
This page is part of release 4.08 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 https://www.kernel.org/doc/man-pages/.