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

[Jian Lin]

2009/11/23 Pádraig Brady <address@hidden>:
> 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?

I think so. I run md5sum immediately after cloning the file.

> 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");
>  }
>  close(in);
>  close(out);
>  return 0;
> }

I compiled and tried this program.
./clone WinXP_CHS.vmdk WinXP_CHS-clone.vmdk
then I got md5 of WinXP_CHS-clone.vmdk, which is the same as that of
WinXP_CHS-ref.vmdk (made by cp --reflink), and different from the md5
of original WinXP_CHS.vmdk.

The cloned vm cannot run correctly, which said "hal.dll lost". The
cloned Linux vm I mentioned before also reported "/bin/bash lost".
Another tiny linux vm I cloned with the same md5 to its original vmdk
can run correctly.

>
> cheers,
> Pádraig.
>

Jian LIN