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Re: A file cloned with "cp --reflink" different from the original one?

From: Pádraig Brady
Subject: Re: A file cloned with "cp --reflink" different from the original one?
Date: Mon, 23 Nov 2009 13:36:01 +0000
User-agent: Thunderbird (X11/20071008)

Jian Lin wrote:
> I installed BtrFS 0.19 and GNU coreutils 8.1 on my Ubuntu 9.10.
> I tried to clone some files with "cp --reflink" to make them "copy-on-write".
> However, I found some of the files cloned have different MD5s to the
> original one.
> Is BtrFS (or cp with reflink) buggy?
> Or it is indeed a feature that I used incorrectly?
> Thanks!
> address@hidden:/mnt# uname -a
> Linux lj-laptop 2.6.31-15-generic #50-Ubuntu SMP Tue Nov 10 14:54:29
> UTC 2009 i686 GNU/Linux
> address@hidden:/mnt# mount | grep btrfs
> /dev/sda4 on /mnt type btrfs (rw)
> address@hidden:/mnt# cp --version
> cp (GNU coreutils) 8.1
> address@hidden:/mnt# cd WinXP_CHS/
> address@hidden:/mnt/WinXP_CHS# cp WinXP_CHS.vmdk WinXP_CHS-ref.vmdk --reflink
> address@hidden:/mnt/WinXP_CHS# ls -la WinXP_CHS.vmdk WinXP_CHS-ref.vmdk
> -rwxr-xr-x 1 root root 1713766400 Nov 23 15:11 WinXP_CHS-ref.vmdk
> -rwxr-xr-x 1 root root 1713766400 Nov 23 13:32 WinXP_CHS.vmdk
> address@hidden:/mnt/WinXP_CHS# md5sum WinXP_CHS.vmdk WinXP_CHS-ref.vmdk
> f6da592d056167a44b089d70fa46f863  WinXP_CHS.vmdk
> e3ee0c2c17771811c80eed088c20987d  WinXP_CHS-ref.vmdk

Yikes. Are you 100% sure nothing is changing those vm files?
Note I wouldn't 100% trust the mtime, so an md5sum before
and after would be good to confirm.

You could try this simple prog to do the clones to
implicate coreutils or otherwise:

#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#define BTRFS_IOC_CLONE 1074041865

int main(int argc, const char **argv)
  int in = open(argv[1], O_RDONLY);
  int out = open(argv[2], O_CREAT|O_TRUNC|O_WRONLY, 0644);
  if (ioctl(out, BTRFS_IOC_CLONE, in)==-1) {
      fprintf(stderr, "ioctl error = %m\n");
  return 0;


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