[Lxc-users] Container size minialisation

[Zhu Yanhai]

2011/12/13 Ivan Vilata i Balaguer <ivan at selidor.net>:
> Zhu Yanhai (2011-12-13 12:00:20 +0800) wrote:
>
>> > On 12/12/2011 09:47 AM, István Király - LaKing wrote:
>> >
>> > As next step I would like to minimize container size. My question is, what
>> > the best, most elegant and fail proof  technique for that?
>>
>> It's not only an issue about disk space, it's about page cache cost.  If the
>> under level files are shared, the OS see them as unique inodes, so the OS
>> allocate only one copy for each one inode. However if we copy them as
>> separate directories, each of them will occupy one copy in the page cache,
>> resulting in large cost of memory.  I think a better approach here is a
>> file-level copy-on-write trick in filesystem or some other proper
>> layers. Initially all files are shared in the host, and copy one as a
>> private item at the first time it is touched in one containers.
>
> I've carried out some tests with union mount points and Btrfs seeds and CoW
> copies, and all of them seem to work without problems with LXC.
>
> For union mount points, you can keep a base rootfs under a read-only
> directory, and then mount a writable directory on top of that for each
> container's rootfs, like this:
>
>  /var/lib/lxc/.base/debian-squeeze (RO, don't change it!)
>  + /var/lib/lxc/.store/lxc01/ (RW) = /var/lib/lxc/lxc01/rootfs/ (RW)
>  + /var/lib/lxc/.store/lxc02/ (RW) = /var/lib/lxc/lxc02/rootfs/ (RW)
>
> Changed files are written under ".store/lxcNN".  Debian includes AuFS for
> union mount points.
>
> A similar solution are Btrfs seeds: you have a read-only seed Btrfs and
> writable images (e.g. files via loop devices) on top, like this:
>
>  /dev/loop0 (var/lib/lxc/.base/debian-squeeze.btrfs) (RO, seed)
>  + /dev/loop1 (/var/lib/lxc/.store/lxc01.raw) (RW) = /var/lib/lxc/lxc01/rootfs/ (RW)
>  + /dev/loop2 (/var/lib/lxc/.store/lxc02.raw) (RW) = /var/lib/lxc/lxc02/rootfs/ (RW)
>
> Changed blocks are stored under ".store/lxcNN.raw" (which should be big
> enough).
>
> As for CoW copies in Btrfs, you can use simple CoW copies:
>
>  # cp -a --reflink /var/lib/lxc/.base/debian-squeeze /var/lib/lxc/lxc01/rootfs/
>
> or you can create a subvolume out of /var/lib/lxc/.base/debian-squeeze in your
> Btrfs filesystem and /var/lib/lxc/lxc01/rootfs/ as a snapshot of that
> subvolume.  In both cases only changed blocks are stored on disc, but I've
> found subvolumes to be a lot faster on creation and removal.
>
> Hope that helps,
> --
> Ivan Vilata i Balaguer -- https://elvil.net/

Thank you Ivan.
My concern is deploying Btrfs only for COW is a really heavy solution
for this...Is Btrfs ready for production system?

-zyh