Why isnt NOCOW attributes propogated on snapshot transfers?

Why isnt NOCOW attributes propogated on snapshot transfers?

[Cerem Cem ASLAN]

`btrfs send | btrfs receive` removes NOCOW attributes. Is it a bug or
a feature? If it's a feature, how can we keep these attributes if we
need to?

Re: Why isnt NOCOW attributes propogated on snapshot transfers?

[Qu Wenruo]

On 2017年10月15日 09:19, Cerem Cem ASLAN wrote:
> `btrfs send | btrfs receive` removes NOCOW attributes. Is it a bug or
> a feature? If it's a feature, how can we keep these attributes if we
> need to?

It seems that, current send doesn't have support for extra inode flags.

Send can only send out c/m/atime, uid/gid and xattr.

Since NODATACOW is stored in inode item flags, not xattr, so it's not 
supported yet.

Thanks,
Qu
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Re: Why isnt NOCOW attributes propogated on snapshot transfers?

[David Sterba]

On Sun, Oct 15, 2017 at 04:19:23AM +0300, Cerem Cem ASLAN wrote:
> `btrfs send | btrfs receive` removes NOCOW attributes. Is it a bug or
> a feature? If it's a feature, how can we keep these attributes if we
> need to?

This is a known defficiency of send protocol v1. And there are more,
listed on
https://btrfs.wiki.kernel.org/index.php/Design_notes_on_Send/Receive#Send_stream_v2_draft

Re: Why isnt NOCOW attributes propogated on snapshot transfers?

[Graham Cobb]

On 16/10/17 14:28, David Sterba wrote:
> On Sun, Oct 15, 2017 at 04:19:23AM +0300, Cerem Cem ASLAN wrote:
>> `btrfs send | btrfs receive` removes NOCOW attributes. Is it a bug or
>> a feature? If it's a feature, how can we keep these attributes if we
>> need to?
> 
> This is a known defficiency of send protocol v1. And there are more,
> listed on
> https://btrfs.wiki.kernel.org/index.php/Design_notes_on_Send/Receive#Send_stream_v2_draft

It is not mentioned on the list (and I haven't tested to find out)...
but if xattr are supported in the V1 protocol, any chance of `send`
converting these file flags to xattr, and `receive` converting back? No
need for a protocol bump and would even preserve the information
(although not the effect) in the case of an old receiver.