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

[Daniel Lezcano]

On 06/09/2010 10:46 PM, Gordan Bobic wrote:
> 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?
>    

Yes.

> 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.
>    

The use case you are describing suppose the guests do not upgrade their 
os, so no need of a cow fs for some private modifications, no ?

In this case, an empty file hierarchy as a rootfs and the hosts system 
libraries, tools directories can be ro-binded-mounted in this rootfs 
with a private /etc and /home.

So when you upgrade the host, all the containers are upgraded too.

Of course, a separate rootfs template can be used instead of the host fs.

In the lxc(7) man there is an example (keyword sshd).

http://lxc.sourceforge.net/man/lxc.html

   -- Daniel