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So far in this book we have focused primarily on the creation of Xen guest domains (domainU). By this stage it is safe to assume that you now have one or more domainU systems up and running on your server or desktop. Given this assumption, this chapter of [[Xen Virtualization Essentials]] will be dedicated to providing an overview of  the tools and techniques that may be employed to monitor a Xen based environment.

== Why Monitor a Xen Environment? ==

It is important to keep in mind that Xen is an enterprise level environment capable of supporting complex virtualization configurations. As with any complex system it would be naive to assume that the system will run without the occasional performance issues or problem. Deploying Xen virtualization therefore requires an understanding of the tools and techniques necessary to monitor the running environment, identify performance issues and track down problems.

== Obtaining Xen Configuration and System Information ==

Perhaps the most basic step in monitoring a Xen system or isolating a problem is to get a high level overview of the Xen environment and underlying configuration. This information will be of particular importance when requesting help from a vendor or forum. A good way to obtain this information is to use the ''xm info'' command. For example, the following example shows output from ''xm info'' on a Red Hat Enterprise Linux 5 (RHEL5) system:

<pre>
xm info
host                   : localhost.localdomain
release                : 2.6.18-53.el5xen
version                : #1 SMP Wed Oct 10 17:06:12 EDT 2007
machine                : i686
nr_cpus                : 1
nr_nodes               : 1
sockets_per_node       : 1
cores_per_socket       : 1
threads_per_core       : 1
cpu_mhz                : 2993
hw_caps                : 0febfbff:20100000:00000000:00000180:0000a015:00000000:00000001
total_memory           : 255
free_memory            : 14
xen_major              : 3
xen_minor              : 1
xen_extra              : .0-53.el5
xen_caps               : xen-3.0-x86_32p 
xen_pagesize           : 4096
platform_params        : virt_start=0xf5800000
xen_changeset          : unavailable
cc_compiler            : gcc version 4.1.2 20070626 (Red Hat 4.1.2-14)
cc_compile_by          : brewbuilder
cc_compile_domain      : build.redhat.com
cc_compile_date        : Wed Oct 10 16:30:55 EDT 2007
xend_config_format     : 2
</pre>


== Monitoring Xen Performance with XenMon ==

The ''XenMon'' tool is useful for monitoring the performance Xen domains, particularly when identifying which domains are responsible for the highest I/O or processing loads on a system.

XenMon is started from the command-line using the ''xenmon.py'' command. The following figure shows a typical XenMon session:

[[Image:xenmon_display.jpg|Monitoring Xen Performance with XenMon]]

The XenMon display shows two sets of data. On the left hand side are statistics captured over the preceding 10 seconds and on the right is the data for the last 1 second. 

For each domain three sets of data are provided. The first row (the grammatically dubious ''Gotten'') for each domain is the amount of time the domain as spent executing. The ''Blocked'' row shows statistics for idle time. Finally, the ''Waited'' row indicates the amount of time the domain has been in a wait state. For each category the amount of time spent in the particular mode and the time as a percentage of overall time during the corresponding period (i.e 1 or 10 seconds) is displayed. The final value depends on the category. For Gotten this represents processor time, for Blocked the average blocked time and for Wait the average waiting time.

By default XenMon displays information for CPU 0. If the system has more than one physical CPU then the '''p''' and '''n''' keys can be used to page through the data for each CPU on  the system. 

When XenMon is exited (using the '''q''' key) a summary of data collected during the monitoring session is displayed:

<pre>
ms_per_sample = 100
Initialized with 1 cpu
CPU Frequency = 2993.98
Event counts:
00000000        Other
00000000        Add Domain
00000000        Remove Domain
00000000        Sleep
00022838        Wake
00022838        Block
00045666        Switch
00000000        Timer Func
00045666        Switch Prev
00045666        Switch Next
00000000        Page Map
00000000        Page Unmap
00000000        Page Transfer
processed 182674 total records in 288 seconds (634 per second)
woke up 288 times in 288 seconds (1 per second)
</pre>

XenMon accepts a range of command-line options which control various aspects of the monitoring. For a breakdown of these options simply pass the ''--help'' argument through to ''xenmon.py'':

<pre>
xenmon.py --help
usage: xenmon.py [options]

options:
  -h, --help            show this help message and exit
  -l, --live            show the ncurses live monitoring frontend (default)
  -n, --notlive         write to file instead of live monitoring
  -p PREFIX, --prefix=PREFIX
                        prefix to use for output files
  -t DURATION, --time=DURATION
                        stop logging to file after this much time has elapsed
                        (in seconds). set to 0 to keep logging indefinitely
  -i INTERVAL, --interval=INTERVAL
                        interval for logging (in ms)
  --ms_per_sample=MSPERSAMPLE
                        determines how many ms worth of data goes in a sample
  --cpu=CPU             specifies which cpu to display data for
  --allocated           Display allocated time for each domain
  --noallocated         Don't display allocated time for each domain
  --blocked             Display blocked time for each domain
  --noblocked           Don't display blocked time for each domain
  --waited              Display waiting time for each domain
  --nowaited            Don't display waiting time for each domain
  --excount             Display execution count for each domain
  --noexcount           Don't display execution count for each domain
  --iocount             Display I/O count for each domain
  --noiocount           Don't display I/O count for each domain
</pre>

== Monitoring Performance with XenTop ==

Anyone who has been using UNIX or Linux for any length of time (particularly since the days before GUI desktop environments) is probably familiar with the ''top'' command. This long standing tool is used to display information, such as CPU and memory usage, about processes running on a particular system. One of the best features of ''top'' is that is puts the process making the heaviest use of a particular resource at the top of the list. When a system is exhibiting performance degradation the ''top'' command is often the first port of call for the experienced system administrator.

''XenTop'' is essentially a Xen version of the original ''top'' utility and is used to show information about all the domains running on a particular system.

The ''XenTop'' tool is launched by typing ''xentop'' as root at the command-line. Whilst xentop can be launched without any command-line options, it is worth knowing that a range of options are available and can be listed using the --help flag:

<pre>
xentop --help
Usage: xentop [OPTION]
Displays ongoing information about xen vm resources 

-h, --help           display this help and exit
-V, --version        output version information and exit
-d, --delay=SECONDS  seconds between updates (default 3)
-n, --networks       output vif network data
-x, --vbds           output vbd block device data
-r, --repeat-header  repeat table header before each domain
-v, --vcpus          output vcpu data
-b, --batch          output in batch mode, no user input accepted
-i, --iterations     number of iterations before exiting

Report bugs to <dsteklof@us.ibm.com>.
</pre>

The following figure shows sample output from a XenTop session:

[[Image:XenTop_display.jpg|Monitoring Xen Domains with XenTop]]


<hr>
<table border="0" cellspacing="0" width="100%">
<tr>
<td width="20%">[[Adding Disk, CDROM and DVD Devices to a Running Xen domainU Guest System|Previous]]<td align="center">[[Xen Virtualization Essentials|Table of Contents]]<td width="20%" align="right">[[Migrating Xen domainU Guests Between Host Systems|Next]]</td>
<tr>
<td width="20%">Adding Disk, CDROM and DVD Devices to a Running Xen domainU Guest System<td align="center"><td width="20%" align="right">Migrating Xen domainU Guests Between Host Systems</td>