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From: Gregory (Grisha) Trubetskoy (grisha_at_ispol.com)
Date: Tue 06 Jan 2004 - 02:56:00 GMT

Hi -

I've been using FreeBSD jail for a while, but am new to Vserver, which
seems like cool stuff, I'm thinking of using it for hosting customers.
I've got a few questions for the list.

1. On FreeBSD, a common way to share files across virtual servers is to
mount them using unionfs read-only, which allows basically one part of a
file-system to be mounted in another place.

The idea is to then symlink all important files shared across jails to
that mount point to save space. The user of the virtual server can then
delete the symlinks as he wishes to replace them with real files (which
will take up more space).

First, there doesn't seem to be anything like unionfs on Linux (mount
--bind doesn't really allow read-only mounting). I'm thinking NFS might do
the trick, though.

Has anyone tried a set up like this, or are people generally happy with
using the hard links as described in the vserver docs? It seems to me
there is an advantage to symlinks in that you can modify the files that
the links point to and the change is immediate across all virtual servers.
Plus, symlinks make it a bit more apparent to the user on what's going on
(although sometimes you may not want this) and make space accounting

2. Another thing common in the BSD world is to place jails into vnode file
systems - i.e. a filesystem mounted from an image file, same as the mount
loopback on Linux. This is a pretty efficient way to restrict disk space,
but, of course you won't be able to use hard links in this set up since
hard links cannot cross filesystems.

The only other alternative to restricting space seems to be the context
quota patch - does this work reliably? What does df show from within the

Basically I'm a bit torn between trying to set up virtual servers using
the ways I learned in FreeBSD - loopback filesystem, nfs mounted read-only
directory with symlinks, or giving up on that idea and just doing using
the context quota and the hard links as described in the vserver faqs.

Has anyone gone through this experience?


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