Re: [PATCH 1/2] man2: document FICLONE and FICLONERANGE

[walter harms <wharms-fPG8STNUNVg@public.gmane.org>]

hi,
i tried to understand the man page and had some problems
(maybe because i am not an FS expert)

So far i understand the use case is to make a virtual copy
of a file (or range). but it works only on the same fs.

reading this:
https://lwn.net/Articles/331576/

I got the impression there is already a syscall ?
Since when is this function available ?

Perhaps you can add a simple use case so readers get a better
idea how/why to use this ?

just my 2 cents,
re,
 wh


Am 15.03.2016 17:48, schrieb Darrick J. Wong:
> Document the FICLONE and FICLONERANGE ioctls, formerly known as the
> BTRFS_IOC_CLONE and BTRFS_IOC_CLONE_RANGE ioctls.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> ---
>  man2/ioctl_ficlone.2      |    1 
>  man2/ioctl_ficlonerange.2 |  124 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 125 insertions(+)
>  create mode 100644 man2/ioctl_ficlone.2
>  create mode 100644 man2/ioctl_ficlonerange.2
> 
> 
> diff --git a/man2/ioctl_ficlone.2 b/man2/ioctl_ficlone.2
> new file mode 100644
> index 0000000..19bb348
> --- /dev/null
> +++ b/man2/ioctl_ficlone.2
> @@ -0,0 +1 @@
> +.so man2/ioctl_ficlonerange.2
> diff --git a/man2/ioctl_ficlonerange.2 b/man2/ioctl_ficlonerange.2
> new file mode 100644
> index 0000000..c5c72aa
> --- /dev/null
> +++ b/man2/ioctl_ficlonerange.2
> @@ -0,0 +1,124 @@
> +.\" Copyright (C) 2016 Oracle.  All rights reserved.
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" This program is free software; you can redistribute it and/or
> +.\" modify it under the terms of the GNU General Public License as
> +.\" published by the Free Software Foundation.
> +.\"
> +.\" This program is distributed in the hope that it would be useful,
> +.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
> +.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +.\" GNU General Public License for more details.
> +.\"
> +.\" You should have received a copy of the GNU General Public License
> +.\" along with this program; if not, write the Free Software Foundation,
> +.\" Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
> +.\" %%%LICENSE_END
> +.TH IOCTL-FICLONERANGE 2 2016-02-10 "Linux" "Linux Programmer's Manual"
> +.SH NAME
> +ioctl_ficlonerange, ioctl_ficlone \- 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 " dest_fd ", FICLONERANGE, struct file_clone_range * " arg );
> +.br
> +.BI "int ioctl(int " dest_fd ", FICLONE, int " src_fd );
> +.SH DESCRIPTION
> +If a filesystem supports files sharing physical storage between multiple
> +files ("reflink"), this
> +.BR ioctl (2)
> +system call 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, which is faster than making a separate
> +physical 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 reflinks up to
> +.IR src_length
> +bytes from file descriptor
> +.IR src_fd
> +at offset
> +.IR src_offset
> +into the file
> +.IR dest_fd
> +at offset
> +.IR dest_offset ",
> +provided that both are files.  This information is conveyed in a structure of
> +the following form:
> +.in +4n
> +.nf
> +
> +struct file_clone_range {
> +        __s64 src_fd;
> +        __u64 src_offset;
> +        __u64 src_length;
> +        __u64 dest_offset;
> +};
> +
> +.fi
> +.in
> +Clones are atomic with regards to concurrent writes, so no locks need to be
> +taken to obtain a consistent cloned copy.
> +
> +The FICLONE ioctl clones entire files.
> +.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 reflinking 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.  XFS and btrfs do not support
> +overlapping reflink 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 reflink.
> +.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 reflinking either file
> +descriptor.
> +.SH NOTES
> +Because a copy on write operation requires the allocation of new storage, the
> +.B fallocate (2)
> +operation may un-share shared blocks to guarantee that subsequent writes will
> +not fail because of lack of disk space.
> +.SH CONFORMING TO
> +This API is Linux-specific.  This ioctl was previously known as
> +.B BTRFS_IOC_CLONE_RANGE
> +and was private to btrfs.
> +.fi
> +.in
> +.SH SEE ALSO
> +.BR ioctl (2)
> 
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