Re: Copy-on-write hard-links

[Gordan Bobic <gordan@bobich.net>]

On 06/10/2010 09:00 PM, Chris Mason wrote:
> On Thu, Jun 10, 2010 at 06:11:40PM +0100, Gordan Bobic wrote:
>> Is there a feature in btrfs to manually/explicitly mark hard-links
>> to be copy-on-write? My understanding is that this is what happens
>> when a snapshot is mounted rw and files modified.
>>
>> Consider this scenario:
>>
>> I have a base template fs. I make two snapshots of it that are
>> identical. The files in the template and both snapshots are
>> hard-links and have the same inode number.
>>
>> I change a file in one of the snapshots, and it gets copied on
>> write. I make the same change in the other snapshot, and that, too,
>> gets copied on write. I now have two identical files that are not
>> hard-links any more.
>>
>> What happens if I remove one of those files and create a hard-link
>> to the file in the other snapshot?
>
> I'm afraid you can't do this.  hard linking between subvolumes isn't
> allowed.  But, what you can do is use the clone ioctl to make a new
> inode that references all of the data extents of an existing file, which
> would be a kind of COW hard link.
>
> Checkout bcp from btrfs-progs or cp --reflink from the latest..well
> wherever cp comes from.

Would the inodes on the clone file be the same for purposes of loading a 
dynamic library? Specifically, say the file I am cloning is a DLL. 
Normally, if a DLL is hard-linked, if two programs dynamically load it 
from two different hard-links, it'll still only use one bit of shared 
memory. Will this also hold true for the cloned files? My understanding 
is that it will not since it's not the same inode. Is that the case?

Thanks.

Gordan