Re: 'cp -a' does not preserve nocow xattrs on btrfs

[A L]

On 2020-05-16 13:31, A L wrote:
> Hi,
>
> Normally, 'cp -a' should preserve xattrs when copying files and folders.
>
> From the man page(*):
> -a, --archive
>               same as -dR --preserve=all
> --preserve[=ATTR_LIST]
>               preserve  the  specified  attributes  (default: 
> mode,ownership,timestamps),  if  possible  additional
>               attributes: context, links, xattr, all
>
>
> There are two important xattrs(*) on the Btrfs filesystem; 'no 
> copy-on-writes' ('+C') and 'compression' (+c). They have specific 
> conditions to work.
>
> The nocow '+C' attribute can only be set on empty files.
> The compression '+c' can be set on any file, but only newly written 
> data will be compressed.
>
> The problem is that 'cp -a' seems to set the xattrs after creating the 
> file and writing the data to it, which means that the nocow '+C' flags 
> can't be set and that files would not be fully compressed, even though 
> the flag is set.
>
> I think that these xattrs should be preserved when doing 'cp -a'.
>
>
>
> * http://man7.org/linux/man-pages/man1/cp.1.html
> * http://man7.org/linux/man-pages/man1/chattr.1.html

I realise I had confused the difference between fattr (aka ext2 
attributes) and xattrs (extended attributes). In Btrfs, the nocow '+C' 
is only available as fattr, and not a xattr, which is why it is  not 
copied with cp. Apart from that, the issue still stands with compression 
flag '+c', since that is also implemented as an xattr 
("btrfs.compression") . cp ought to set the xattrs before writing any 
data to the file, possibly with an fsync in between.

Side note: Why is it that cp -a does not copy file attrs and only 
xattrs? As a end user, the naming similarity fattr and xattr is somewhat 
confusing, and the man page does not clearly state that only the latter 
is supported in cp. http://man7.org/linux/man-pages/man1/cp.1.html

Thank you