Re: Optional switching off cow in overlayfs

[Jan Olszak <j.olszak@samsung.com>]

On 06/22/2015 03:53 PM, Miklos Szeredi wrote:
> On Mon, Jun 22, 2015 at 3:45 PM, Jan Olszak <j.olszak@samsung.com> wrote:
>
>> The lower fs is most likely ext4.
>> Unfortunately making a file immutable won't solve the problem. There's
>> nothing wrong in modifying the file and it should stay possible.
>>
>> I just want to switch off copy on write for some files. Can you see another
>> way?
> Ah, you want to allow modification of file on the lower layer?
Exactly
> You can bind mount individual files from the lower layer to the
> overlay.  That fixes the "allow modification" part.
Well bind mounting every file that should have COW is unmaintainable - 
if new files appear admin has to mount each one.
Plus all the unnecessary mounts. It's much easier to do it the opposite 
way - by default have COW and only for some files switch it off.

>   But what should happen on rename or unlink?
Both would operate on the original inode.


Setting "trusted.overlay.cow" is a privileged operation so it's admin's 
decision to allow this.

Each layer could potentially make the a file COW again by 
setting/removing "trusted.overlay.cow" attr.


Thanks,
Jan