[Samba] Enable XAT_OFFLINE extended attribute in Samba

[David Disseldorp]

On Tue, 5 Mar 2019 11:28:25 -0800, Jeremy Allison via samba wrote:

> Got it. Currently, Samba uses the dmapi API to
> check if a file has been migrated to external
> storage, and to cope with reads/writes against
> it.
> 
> Look at the VFS module source3/modules/vfs_tsmsm.c
> which, as decribed, is:
> 
>   Samba VFS module for handling offline files
>   with Tivoli Storage Manager Space Management
> 
> You would need to create a similar VFS module
> on ZFS to expose the FILE_ATTRIBUTE_OFFLINE
> flag to the client. Mostly it'll be wrapping
> the [f]get_dos_attributes() VFS calls.

IIUC, DMAPI will never make it into mainline. It'd be good if we could
settle on a standard xattr or statx flag for offline files on Linux /
BSD, which could then be used by FSes offering some form of HSM.
Extending user.DOSATTRIB might be an option, but wouldn't be too
friendly to other potential external consumers.

Cheers, David