will this clever plan work?

will this clever plan work?

[Eric S. Johansson]

(originally posted via gmame but never showed up)

I thought of two applications for Xen that could be amusing.  First is 
as a base for a firewall.  The idea would be that in order to upgrade to 
the next release of the firewall, you would create a new virtual machine 
image and disable (but not necessarily expire) the previous version.

Ideally these would be complete system images that one could just add to 
and replace as necessary.  I do hope it's also possible for a linux 
system to change the xen configuration so it could boot to the new 
configuration.

Second I have another project would require running a Gentoo based 
system that can handle multiple virtual instances (maybe as many as 10 
or 20) of a stock application configuration.

My fantasy is that I will be able to build a single disk image for all
of the basic OS and application to be shared between multiple virtual
machines.  Update that single core image using Gentoo magic and be able
to have separate partitions holding my application data.

in the test case, I'm running with a couple of SCSI disks for the OS and
a raid 1 (79 GB real storage) for holding all the virtual system "disk
images".

real?  Fantasy?  Roll up my sleeves and work really really hard,
documenting as I go to make somebody else's life easier?

don't worry about disappointing me.  I have already been disappointed
with EVMS but only enough to make me go away for six months.  ;-)

thank you for whatever information you give me.

---eric


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Re: will this clever plan work?

[Mark A. Williamson]

> I thought of two applications for Xen that could be amusing.  First is
> as a base for a firewall.  The idea would be that in order to upgrade to
> the next release of the firewall, you would create a new virtual machine
> image and disable (but not necessarily expire) the previous version.

It should be possible to build something like that on top of Xen.  It'd be 
quite cute too ;-)

> Ideally these would be complete system images that one could just add to
> and replace as necessary.  I do hope it's also possible for a linux
> system to change the xen configuration so it could boot to the new
> configuration.

One nice way to set this up would be:
* admin software in dom0 (accessed via console or dedicated NIC)
* a separate domain for the firewall software, controlling the NICs directly
* when you upgrade, kill the old domain and build a new one (this can be 
scripted and should be a very quick process with minimal downtime)

> My fantasy is that I will be able to build a single disk image for all
> of the basic OS and application to be shared between multiple virtual
> machines.  Update that single core image using Gentoo magic and be able
> to have separate partitions holding my application data.

Yes, you should be able to do that.  If you're using VBDs (not NFS), the 
standard caveats for shared data apply:
* domains can't write to the shared data
* you can't update the shared data while the domains are running (otherwise 
you'll confuse them)

$0.02,
Mark


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Re: will this clever plan work?

[Eric S. Johansson]

Mark A. Williamson wrote:
>>I thought of two applications for Xen that could be amusing.  First is
>>as a base for a firewall.  The idea would be that in order to upgrade to
>>the next release of the firewall, you would create a new virtual machine
>>image and disable (but not necessarily expire) the previous version.
> 
> 
> It should be possible to build something like that on top of Xen.  It'd be 
> quite cute too ;-)

and if I could make it run out of flash memory, it would be the cat's ass[1]

  > One nice way to set this up would be:
> * admin software in dom0 (accessed via console or dedicated NIC)
> * a separate domain for the firewall software, controlling the NICs directly
> * when you upgrade, kill the old domain and build a new one (this can be 
> scripted and should be a very quick process with minimal downtime)

can the two domains communicate over a virtual NIC?  the reason I ask is 
that since most of the control is by a Web interface, we would need to 
tickle the control system in dom0 or at least proxy to it.


>>My fantasy is that I will be able to build a single disk image for all
>>of the basic OS and application to be shared between multiple virtual
>>machines.  Update that single core image using Gentoo magic and be able
>>to have separate partitions holding my application data.
> 
> 
> Yes, you should be able to do that.  If you're using VBDs (not NFS), the 
> standard caveats for shared data apply:
> * domains can't write to the shared data
> * you can't update the shared data while the domains are running (otherwise 
> you'll confuse them)

I really need to learn how the whole storage metaphor is organized..  I 
don't know enough to ask the right questions yet.  I probably should 
just set up a system with a real standard disk image and partitioning 
and start breaking it.

on a humorous aside, VBD used to refer to people who are so insecure in 
their manhood that they used proxies like expensive cars, trophy wives, 
etc. to show that they had a VBD.

---eric


[1] well, since our feline buddies are always waving their butts in our 
faces like it was the best thing on the face the planet, if it's good, 
it must be the cat's ass  ;-)

-- 
President Nixon: Now more than ever


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Re: will this clever plan work?

[Mark A. Williamson]

> and if I could make it run out of flash memory, it would be the cat's
> ass[1]

You can also pull tricks like running the firewall out of a ramdisk or using a 
CoW LVM VBD so that you can be sure that you can always roll its state back 
to a known config (e.g. after a hacking attempt).

> can the two domains communicate over a virtual NIC?  the reason I ask is
> that since most of the control is by a Web interface, we would need to
> tickle the control system in dom0 or at least proxy to it.

Oooh, are you using xensv (the pretty one)?  Or the Xend web interface?  How's 
it working for you?  I think most people use the xm tool at the console or 
via SSH.

I'm not sure that you can currently get dom0 to use a virtual NIC but it's 
certainly technically possible with a little hacking.  Using a dedicated 
management NIC to talk to dom0 would work right now and has the advantage 
that you could access it even if you exploded your firewall VM ;-)

> I really need to learn how the whole storage metaphor is organized..  I
> don't know enough to ask the right questions yet.  I probably should
> just set up a system with a real standard disk image and partitioning
> and start breaking it.

Linux generally doesn't expect to have filesystems modified underneath it, so 
(unless you're using a cluster filesystem like GFS, etc) Linux will get very 
confused if another domain modifies a filesystem it had been using.  If two 
Linux domains both write to a filesystem then you're certainly going to hose 
it as well as confusing both kernels.

In general, sharing block devices should *only* be read-only by all clients 
unless you're using a cluster FS.  Xend generally shouts at you if you try to 
do something it thinks is unsafe (unless you force the operation).

NFS has support for shared write access, with the server managing consistency 
of the metadata so you can share NFS filesystems safely.

> on a humorous aside, VBD used to refer to people who are so insecure in
> their manhood that they used proxies like expensive cars, trophy wives,
> etc. to show that they had a VBD.

That interpretation of the acronym hadn't occurred to me before!

Cheers,
Mark


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will this clever plan work?

[Eric S. Johansson]

allow me to play someone with a fractional clue for the moment.

What I want is a Gentoo based system that can run multiple virtual 
instances (maybe as many as 10 or 20) of a stock application configuration.

My fantasy is that I will be able to build a single disk image for all 
of the basic OS and application to be shared between multiple virtual 
machines.  Update that single core image using Gentoo magic and be able 
to have separate partitions holding my application data.

in the test case, I'm running with a couple of SCSI disks for the OS and 
a raid 1 (79 GB real storage) for holding all the virtual system "disk 
images".

real?  Fantasy?  Roll up my sleeves and work really really hard, 
documenting as I go to make somebody else's life easier?

don't worry about disappointing me.  I have already been disappointed 
with EVMS but only enough to make me go away for six months.  ;-)

thank you for whatever information you give me.

---eric



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Re: will this clever plan work?

[Keir Fraser]

We have plans to support efficient replication of filesystem snapshots
amongst multiple VMs, which will support the kind of thing you want to
do (push a given OS + app setup to multiple VMs, and have their writes
go to a 'private version' of the filesystem). It'll probably be a
month or two before anything gets checked in though. :-)

 -- Keir

> allow me to play someone with a fractional clue for the moment.
> 
> What I want is a Gentoo based system that can run multiple virtual 
> instances (maybe as many as 10 or 20) of a stock application configuration.
> 
> My fantasy is that I will be able to build a single disk image for all 
> of the basic OS and application to be shared between multiple virtual 
> machines.  Update that single core image using Gentoo magic and be able 
> to have separate partitions holding my application data.
> 
> in the test case, I'm running with a couple of SCSI disks for the OS and 
> a raid 1 (79 GB real storage) for holding all the virtual system "disk 
> images".
> 
> real?  Fantasy?  Roll up my sleeves and work really really hard, 
> documenting as I go to make somebody else's life easier?
> 
> don't worry about disappointing me.  I have already been disappointed 
> with EVMS but only enough to make me go away for six months.  ;-)
> 
> thank you for whatever information you give me.
> 
> ---eric
> 
> 
> 
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> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel



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Re: will this clever plan work?

[Eric S. Johansson]

Keir Fraser wrote:
> We have plans to support efficient replication of filesystem snapshots
> amongst multiple VMs, which will support the kind of thing you want to
> do (push a given OS + app setup to multiple VMs, and have their writes
> go to a 'private version' of the filesystem). It'll probably be a
> month or two before anything gets checked in though. :-)

thanks for the reply.  I had originally posted this query through gmame 
only to be told that it was members only.  So I signed up for the list, 
reposted etc. etc. and apparently the moderator hadn't noticed my 
request to cancel the other posting in the queue.  No big deal.

another person had kindly answered my other copy of this query and we 
had come up with some good workarounds for the very problem you 
describe.  Given the rate at which I will make progress, it will be a 
race to see if the workarounds or your solution gets done first.  ;-)

I must admit I am very excited about xen.  It seems like a very cool 
form of virtualization.  Almost 20 years ago I was working in operating 
systems for Computervision and so many of the issues you wrestle with 
are very familiar.  If it wasn't for a whole bunch of things too boring 
to go into, this is a project I would love to work on.  But first, I 
have my own baby to finish up (www.camram.org).

---eric

-- 
President Nixon: Now more than ever


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