Re: [RFC][PATCH][cp] btrfs, nocow and cp --reflink

[Forza]

On 2022-05-27 16:00, Pádraig Brady wrote:
> On 25/05/2022 18:05, Goffredo Baroncelli wrote:
> > Hi All,
> >
> > recently I discovered that BTRFS allow to reflink a file only if the 
> > flag FS_NOCOW_FL is the same on both source and destination.
> > In the end of this email I added a patch to "cp" to set the 
> > FS_NOCOW_FL flag according to the source.
> >
> > Even tough this works, I am wondering if this is the expected/the 
> > least surprise behavior by/for any user. This is the reason why this 
> > email is tagged as RFC.
> >
> > Without reflink, the default behavior is that the new file has the 
> > FS_NOCOW_FL flag set according to the parent directory; with this 
> > patch the flag would be the same as the source.
> >
> > I am not sure that this is the correct behviour without warning the 
> > user of this change.
> >
> > Another possibility, is to flip the NOCOW flag only if 
> > --reflink=always is passed.
> >
> > Thoughts ?
>
> This flag corresponds to the 'C' chattr attribute,
> to allow users to explicitly disable CoW on certain files
> or files within certain dirs.
>
> I don't think cp should be overriding that explicit config.  I.e.:
>    cp --reflink=auto => try reflink but fall back to normal copy
>    cp --reflink=always => try reflink and fail if not possible
>
> We would need another option to bypass system config
> (like --reflink=force), however I don't think that's
> appropriate functionality for cp.
>
> thanks,
> Pádraig

The solution is that 'cp' should set +C on the target file before 
appending data to it. I don't think that we need any additional mode, 
but the default should be that '--reflink=auto|always' sets the fsattrs 
in the correct order on the target file. This is important for other 
fsattrs as well, such as +i (immutable), +c (compress) and +m (nocompress).

There is thread from two years about about the same issue:
https://lists.gnu.org/archive/html/coreutils/2020-05/msg00014.html

There is also an existing bug report about this issue:
https://lists.gnu.org/r/bug-coreutils/2021-06/msg00003.html

Thanks,
Forza