[Lxc-users] Copy-on-write hard-link / hashify feature

[Gordan Bobic]

On 06/09/2010 09:08 PM, Daniel Lezcano wrote:
> On 06/09/2010 08:45 PM, Gordan Bobic wrote:
>> Is there a feature that allows unifying identical files between guests
>> via hard-links to save both space and memory (on shared libraries)?
>> VServers has a feature for this called hashify, but I haven't been able
>> to find such a thing in LXC documentation. Is there such a thing?
>>
>> Obviously, I could manually do the searching and hard-linking, but this
>> is dangerous since without the copy-on-write feature for such
>> hard-linked files that VServers provides, it would be dangerous as any
>> guest could change a file on all guests.
>>
>> Is there a way to do this safely with LXC?
>
> No because it is supported by the system with the btrfs cow / snapshot
> file system.
>
> https://btrfs.wiki.kernel.org
>
> You can create your btrfs filesystem, mount it somewhere in your fs,
> install a distro and then make a snapshot, that will result in a
> directory. Assign this directory as the rootfs of your container. For
> each container you want to install, create a snapshot of the initial
> installation and assign each resulting directory for a container.

OK, this obviously saves the disk space. What about shared libraries 
memory conservation? Do the shared files in different snapshots have the 
same inodes?

What about re-merging them after they get out of sync? For example, if I 
yum update, and a new glibc gets onto each of the virtual hosts, they 
will become unshared and each get different inode numbers which will 
cause them to no longer be mmap()-ed as one, thus rapidly increasing the 
memory requirements. Is there a way to merge them back together with the 
approach you are suggesting? I ask because VServer tools handle this 
relatively gracefully, and I see it as a frequently occurring usage pattern.

Gordan