flavor "host": reduce memory consumption by sharing systemd pages across pods

[alban]

Filed after #1788 (comment):

At the moment, rkt compiled with the flavor host copies the binaries systemd, systemd-journald, etc. in stage1 at each rkt run instead of relying on bind mounts or overlay fs. It means each binary in stage1 will have a different inode on each pod, and the memory will not be shared across pods. It is wasting memory when running several pods.

We should try to use bind mounts.

[alban]

@dgonyeo do you have an opinion about this?

[alban]

This should be an issue only on distributions that use the "host" flavor, such as Debian and Fedora.

CoreOS Linux and the official releases don't use the "host" flavor.

[cgonyeo]

Sounds like a reasonable thing to do. I haven't poked at the stage1 code at all, so I don't have much of an idea on what it would take to implement it.

[alban]

The code that does the copy is in installAssets(). I guess that instead of using proj2aci.PrepareAssets(), this could use a new function that does bind mounts instead of copies.

[alban]

Low priority, moving to v1+