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man2: New page documenting FICLONE and FICLONERANGE ioctls
Document the FICLONE and FICLONERANGE ioctls, formerly known as the BTRFS_IOC_CLONE and BTRFS_IOC_CLONE_RANGE ioctls. Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
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Darrick J. Wong
Michael Kerrisk
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.so man2/ioctl_ficlonerange.2
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.\" Copyright (C) 2016 Oracle. All rights reserved.
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" This program is free software; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation.
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.\"
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.\" This program is distributed in the hope that it would be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public License
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.\" along with this program; if not, write the Free Software Foundation,
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.\" Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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.\" %%%LICENSE_END
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.TH IOCTL-FICLONERANGE 2 2016-02-10 "Linux" "Linux Programmer's Manual"
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.SH NAME
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ioctl_ficlonerange, ioctl_ficlone \- share some the data of one file with another file
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.SH SYNOPSIS
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.br
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.B #include <sys/ioctl.h>
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.br
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.B #include <linux/fs.h>
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.sp
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.BI "int ioctl(int " dest_fd ", FICLONERANGE, struct file_clone_range * " arg );
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.br
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.BI "int ioctl(int " dest_fd ", FICLONE, int " src_fd );
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.SH DESCRIPTION
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If a filesystem supports files sharing physical storage between multiple
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files ("reflink"), this
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.BR ioctl (2)
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system call can be used to make some of the data in the
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.B src_fd
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file appear in the
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.B dest_fd
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file by sharing the underlying storage, which is faster than making a separate
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physical copy of the data. If a file write should occur to a shared region,
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the filesystem must ensure that the changes remain private to the file being
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written. This behavior is commonly referred to as "copy on write".
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This ioctl reflinks up to
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.IR src_length
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bytes from file descriptor
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.IR src_fd
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at offset
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.IR src_offset
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into the file
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.IR dest_fd
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at offset
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.IR dest_offset ",
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provided that both are files. This information is conveyed in a structure of
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the following form:
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.in +4n
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.nf
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struct file_clone_range {
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__s64 src_fd;
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__u64 src_offset;
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__u64 src_length;
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__u64 dest_offset;
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};
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.fi
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.in
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Clones are atomic with regards to concurrent writes, so no locks need to be
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taken to obtain a consistent cloned copy.
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The FICLONE ioctl clones entire files.
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.SH RETURN VALUE
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.PP
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.SH ERRORS
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Error codes can be one of, but are not limited to, the following:
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.TP
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.B EXDEV
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.IR dest_fd " and " src_fd
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are not on the same mounted filesystem.
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.TP
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.B EISDIR
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One of the files is a directory and the filesystem does not support shared
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regions in directories.
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.TP
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.B EINVAL
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The filesystem does not support reflinking the ranges of the given files. This
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error can also appear if either file descriptor represents a device, fifo, or
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socket. Disk filesystems generally require the offset and length arguments
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to be aligned to the fundamental block size. XFS and btrfs do not support
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overlapping reflink ranges in the same file.
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.TP
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.B EBADF
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.IR src_fd
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is not open for reading;
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.IR dest_fd
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is not open for writing or is open for append-only writes; or the filesystem
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which
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.IR src_fd
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resides on does not support reflink.
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.TP
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.B EPERM
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.IR dest_fd
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is immutable.
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.TP
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.B ETXTBSY
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One of the files is a swap file. Swap files cannot share storage.
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.TP
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.B EOPNOTSUPP
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This can appear if the filesystem does not support reflinking either file
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descriptor.
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.SH NOTES
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Because a copy on write operation requires the allocation of new storage, the
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.B fallocate (2)
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operation may un-share shared blocks to guarantee that subsequent writes will
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not fail because of lack of disk space.
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.SH CONFORMING TO
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This API is Linux-specific. This ioctl was previously known as
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.B BTRFS_IOC_CLONE_RANGE
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and was private to btrfs.
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.fi
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.in
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.SH SEE ALSO
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.BR ioctl (2)
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