Re: [Xen-users] Multiple Domains Sharing Root System

[Chris de Vidal <chris@xxxxxxxxxx>]

--- Marcus Brown <marcusbrutus@xxxxxxxxxxxxxxxx> wrote:
> My plan is to have an NFS root server as a domain, using multiple LVM
> partitions. (I could run it in dom0 if required).
> Using a UnionFS-patched kernel I hope to combine a (writable) domain-
> specific partition with a (read-only) generic image for each domain.
> This would allow each domain to have custom configs and apps.

As I was contemplating with the COW filesystems, wouldn't a UnionFS over time
grow to be about the same size as if you'd have done individual installs?

For instance, say you have these files in a UnionFS partition:
/usr/lib/mozilla-firefox (NFS server)
/usr/lib/samba (NFS server)
/usr/lib/glibc (NFS server)

You have two NFS clients who mount /usr.

NFS client number one updates Firefox.  Because file writes are copied, this
doubles the file space needed:
/usr/lib/mozilla-firefox (NFS server)
/usr/lib/mozilla-firefox (NFS client one)
/usr/lib/samba (NFS server)
/usr/lib/glibc (NFS server)

NFS client number two updates Firefox.  Because file writes are copied, this
now triples the file space needed:
/usr/lib/mozilla-firefox (NFS server)
/usr/lib/mozilla-firefox (NFS client one)
/usr/lib/mozilla-firefox (NFS client two)
/usr/lib/samba (NFS server)
/usr/lib/glibc (NFS server)

Now you update glibc.  Samba is recompiled for that new version:
/usr/lib/mozilla-firefox (NFS server)
/usr/lib/mozilla-firefox (NFS client one)
/usr/lib/mozilla-firefox (NFS client two)
/usr/lib/samba (NFS server)
/usr/lib/samba (NFS client one)
/usr/lib/samba (NFS client two)
/usr/lib/glibc (NFS server)
/usr/lib/glibc (NFS client one)
/usr/lib/glibc (NFS client two)

Since the NFS server is now redundant, this would use even more space than this
simple back-to-basics configuration:
/usr/lib/mozilla-firefox (system one)
/usr/lib/samba (system one)
/usr/lib/glibc (system one)

/usr/lib/mozilla-firefox (system two)
/usr/lib/samba (system two)
/usr/lib/glibc (system two)


Seems that UnionFS and COW would only save space at first or with files that
are almost never expected to change over time (unlike OS files which get
modified every few months).

Am I missing something?  I've never used COW and I've only used UnionFS in
Knoppix, so I could be wayyyy off...

Is there some simple method to "reconsolidate" the various identical copies in
different file systems to shrink the number of incremental changes?  Is this
automatically done when there are identical files?

CD

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