Re: option to mount read-only subvolume with read-write access

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 10, 2022 at 05:12:52AM -0500, Eric Levy wrote:
> At times it is helpful to modify a subvolume to which has been assigned 
> a read-only property, without changing the property.

So that would be a silent change in the subvolume and not detectable by
eg. checking the generation. That would break incremental send at least
and goes against the point of read-only subvolume. If you'd want to do a
silent change it would have to be something that does not need to
copy-on-write any block (that requires generation change) so it would
have to be in-place and with recalculated checksum.

> One reason for 
> keeping the property is that the same subvolume may be reachable 
> through another mount point. Another is to ensure that the property 
> persists even if some operation fails to complete.

Yeah but the other mount points would see the old data, depending on the
cached status of the blocks.

> Would you consider adding a mount option to cause the system to ignore 
> a read-only property for the subvolume accessed through the mount 
> point, without affecting access to the same subvolume through other 
> mount points?

I think it does not have a well defined semantics, what you ask seems to
be some kind of multi headed subvolume that could appear different
depending on the way it's accessed from the VFS mount point which is out
of reach in many cases.

I'd probably need to understand what you mean in the first sentence "At
times it is helpful", I don't have an idea where it would be helpful.