Commit 8018831e authored by Mike Hibler's avatar Mike Hibler

Still more text, almost done.

parent d544e591
......@@ -189,23 +189,93 @@ anyway so that all virtual links have the same MTU.
<a NAME="AdvancedIssues"></a><h2>Advanced Issues</h2>
<h3>Taking advantage of a virtual node host.</h3>
A physical node hosting one or more virtual nodes is ''invisible'' to the
topology, it exists only to host virtual nodes. You can however use the
physical node in a variety of ways: monitoring, proxying, efficient
distribution of files.
How to find the vnode filesystems,
what is shared from the host,
monitoring veth and phys eth devices,
routing tables.
A physical node hosting one or more virtual nodes is not itself part of
the topology, it exists only to host virtual nodes. However, the physical
node is still setup with user accounts and shared filesystems just as a
regular node is. Thus you can login to, and use the physical node in a
variety of ways:
<ul>
<li> Since the /usr file system for each node is mounted via a read-only
loopback mount from the physical host, any files installed on a
physical host's /usr will automatically be part of every virtual node
as well. This allows for a potentially more efficient file distribution
mechanism:
install packages in the host's /usr and they are visible in the virtual
nodes as well. Unfortunately, there is currently no "handle" for a
virtual node host in the NS file, so you cannot install tarballs or
RPMs on it as part of the experiment creation process. You must install
them by hand after the experiment has been created, and reboot the
virtual nodes. Thereafter, the packages will be available.
<li> The private root filesystem for each virtual node is also accessible
to the host node in
<code>/var/emulab/jails/</code><i>vnodename</i><code>/root</code>
where <i>vnodename</i> is the "pcvmNN-NN" Emulab name. Thus the host
can monitor log files and even change files on the fly.
<li> Other forms of monitoring can be done as well since all processes,
filesystems, network interfaces and routing tables are visible in the
host. For instance, you can run tcpdump on a virtual interface outside
the node rather than inside it. You can also tcpdump on a physical
interface on which many virtual nodes' traffic is multiplexed. The
installed version of tcpdump understands the veth encapsulation.
</ul>
We should emphasize however, that virtual nodes are not "performance
isolated" from each other or from the host; i.e., a big CPU hogging
monitor application in the host might affect the performance and behavior
of the hosted virtual nodes.
<h3>Controlling virtual node layout.</h3>
Using tb-set-colocate-factor,
using tb-fix-node,
<p>
Normally, the Emulab resource mapper, <code>assign</code>
will map virtual nodes onto physical
nodes in such a way as to achieve the best overall use of physical resources
without violating any of the constraints of the virtual nodes or links.
In a nutshell, it packs as many virtual nodes onto a physical node as it
can without exceeding a node's internal or external network bandwidth
capabilities and without exceeding a node-type specific static packing
factor. Internal network bandwidth is an empirically derived value for
how much network data can be moved through internally connected virtual
ethernet interfaces. External network bandwidth is based on the number
of physical interfaces available on the node. The static packing factor is
intended as a coarse metric of CPU and memory load that a physical node
can support, currently it is based strictly on the amount of physical memory.
The current values for these constraints are:
<ul>
<li>Internal network bandwidth: 400Mb/sec for all node types
<li>External network bandwidth: 400Mb/sec for all node types
<li>Packing factor: 10 for pc600s and pc1500s, 20 for pc850s and pc2000s
</ul>
</p><p>
The mapper generally produces an "unsurprising" mapping of virtual nodes
to physical nodes (e.g., mapping small LANs all on the same physical host)
and where it doesn't, it is usually because doing so would violate one
of the constraints. However, there are circumstances in which you might
want to modify or even override the way in which mapping is done.
Currently there are only limited ways in which to do this, and none of
these will allow you to violate the constrains above.
</p><p>
Using the NS-extension <code>tb-set-colocate-factor</code> command, you
can globally reduce (not increase!) the maximum number of virtual nodes
per physical node. This command is useful if you know the application
load you are running in the vnodes is going to require more resources
per instance (e.g., a java DHT).
Note that currently, this is not really a "factor,"
it is an absolute value. Setting it to 5 will reduce the capacity of
all node types to 5, whether they were 10 or 20 by default.
</p><p>
Since <code>assign</code> uses a heuristic algorithm at its core,
sometime it just doesn't find the best solution that you might think
is obvious. If assign just won't colocate virtual nodes that you want
colocated, you can resort to trying to do the mapping by hand using
<code>tb-fix-node</code>.
<i>TODO:
using tb-set-jail-os,
using tb-set-noshaping,
understanding how bandwidth affects layout.
How do I know what the right colocate factor is?
ENDTODO</i>
<h3>Mixing virtual and physical nodes.</h3>
<a NAME="Limitations"></a><h2>Limitations</h2>
<i>TODO:
Must run FreeBSD and a particular version at that.
No resource guarantees for CPU and memory.
veth encapsulation reduces MTU.
......@@ -216,8 +286,11 @@ Only scale to low 1000s of nodes due to various bottlenecks
No consoles.
Always use linkdelays (more overhead, requires 1000Hz kernel).
Not a complete virtualization, many commands "see through".
ENDTODO</i>
<a NAME="KnownBugs"></a><h2>Known Bugs</h2>
<i>TODO:
Deadlocks in loopback mounts.
ENDTODO</i>
<a NAME="TechDetails"></a><h2>Technical Details</h2>
There is an
<a href="../doc/docwrapper.php3?docname=jail.html">online document</a>
......
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