man-pages/man2/ioctl_fideduperange.2

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.TH IOCTL-FIDEDUPERANGE 2 2016-07-17 "Linux" "Linux Programmer's Manual"
.SH NAME
ioctl_fideduperange \- share some the data of one file with another file
.SH SYNOPSIS
.br
.B #include <sys/ioctl.h>
.br
.B #include <linux/fs.h>
.sp
.BI "int ioctl(int " src_fd ", FIDEDUPERANGE, struct file_dedupe_range * " arg );
.SH DESCRIPTION
If a filesystem supports files sharing physical storage between multiple
files, this
.BR ioctl (2)
operation can be used to make some of the data in the
.B src_fd
file appear in the
.B 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
.IR src_length
bytes from file descriptor
.IR src_fd
at offset
.IR src_offset ".
This information is conveyed in a structure of the following form:
.in +4n
.nf

struct file_dedupe_range {
    __u64 src_offset;
    __u64 src_length;
    __u16 dest_count;
    __u16 reserved1;
    __u32 reserved2;
    struct file_dedupe_range_info info[0];
};
.fi
.in
Deduplication is atomic with regards to concurrent writes, so no locks need to
be taken to obtain a consistent deduplicated copy.

The fields
.IR 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
.IR dest_count ",
and the destination information is conveyed in the following form:

.in +4n
.nf
struct file_dedupe_range_info {
        __s64 dest_fd;
        __u64 dest_offset;
        __u64 bytes_deduped;
        __s32 status;
        __u32 reserved;
};

.fi
.in

Each deduplication operation targets
.IR length
bytes in file descriptor
.IR dest_fd
at offset
.IR logical_offset ".
The field
.IR reserved
must be zero.

Upon successful completion of this ioctl, the number of bytes successfully
deduplicated is returned in
.IR bytes_deduped
and a status code for the deduplication operation is returned in
.IR status ".

The
.IR status
code is set to
.B 0
for success, a negative error code in case of error, or
.B FILE_DEDUPE_RANGE_DIFFERS
if the data did not match.

.SH RETURN VALUE
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.PP
.SH ERRORS
Error codes can be one of, but are not limited to, the following:
.TP
.B EXDEV
.IR dest_fd " and " src_fd
are not on the same mounted filesystem.
.TP
.B EISDIR
One of the files is a directory and the filesystem does not support shared
regions in directories.
.TP
.B 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.
.TP
.B EBADF
.IR src_fd
is not open for reading;
.IR dest_fd
is not open for writing or is open for append-only writes; or the filesystem
which
.IR src_fd
resides on does not support deduplication.
.TP
.B EPERM
.IR dest_fd
is immutable.
.TP
.B ETXTBSY
One of the files is a swap file.
Swap files cannot share storage.
.TP
.B EOPNOTSUPP
This can appear if the filesystem does not support deduplicating either file
descriptor.
.SH VERSIONS
This ioctl operation first appeared in Linux 4.5.
It was previously known as
.B BTRFS_IOC_FILE_EXTENT_SAME
and was private to Btrfs.
.SH CONFORMING TO
This API is Linux-specific.
.SH NOTES
Because a copy-on-write operation requires the allocation of new storage, the
.BR 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.
.SH SEE ALSO
.BR ioctl (2)