Running Systems

Unlike VMware Server, Xen’s HyperVisor does not allow an easy collection of performance information. The management machine, called “Domain-0” is actually a privileged virtual machine, and thus – get its own small share of CPUs and RAM. Collecting performance information on it will lead to, well, collecting performance information for a single VM, and not the whole bunch.

Local tools, such as “xentop” allows collection of information, however, combining this with Cacti, or any other SNMP-based collection tool is a bit tricky.

A great solution is provided by Ian P. Christian in his blog post about Xen monitoring. He has created a Perl script to collect information. I have taken the liberty to fix several minor things with his permission. The modified scripts are presented below. Name the script (according to your version of Xen) “/usr/local/bin/xen_stats.pl” and set it to be executable:

For Xen 3.1

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#!/usr/bin/perl -w
 
use strict;
 
# declare…
sub trim($);
 
# we need to run 2 iterations because CPU stats show 0% on the first, and I’m putting .1 second between them to speed it up
my @result = split(/n/, `/usr/sbin/xentop -b -i 2 -d.1`);
 
# remove the first line
shift(@result);
shift(@result) while @result && $result[0] !~ /^[t ]+NAME/;
shift(@result);
 
foreach my $line (@result)
{
        my @xenInfo = split(/[t ]+/, trim($line));
        printf(“name: %s, cpu_sec: %d, cpu_percent: %.2f, vbd_rd: %d, vbd_wr: %dn“,
        $xenInfo[0],
        $xenInfo[2],
        $xenInfo[3],
        $xenInfo[14],
        $xenInfo[15]
        );
}
# trims leading and trailing whitespace
sub trim($)
{
        my $string = shift;
        $string =~ s/^s+//;
        $string =~ s/s+$//;
        return $string;
}

For Xen 3.2 and Xen 3.3

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#!/usr/bin/perl -w
 
use strict;
 
# declare…
sub trim($);
 
# we need to run 2 iterations because CPU stats show 0% on the first, and I’m putting .1 second between them to speed it up
my @result = split(/n/, `/usr/sbin/xentop -b -i 2 -d.1`);
 
# remove the first line
shift(@result);
shift(@result) while @result && $result[0] !~ /^[t ]+NAME/;
shift(@result);
 
foreach my $line (@result)
{
        my @xenInfo = split(/[t ]+/, trim($line));
        printf(“name: %s, cpu_sec: %d, cpu_percent: %.2f, vbd_rd: %d, vbd_wr: %dn“,
        $xenInfo[0],
        $xenInfo[2],
        $xenInfo[3],
        $xenInfo[14],
        $xenInfo[15]
        );
}
# trims leading and trailing whitespace
sub trim($)
{
        my $string = shift;
        $string =~ s/^s+//;
        $string =~ s/s+$//;
        return $string;
}

Cron settings for Domain-0

Create a file “/etc/cron.d/xenstat” with the following contents:

# This will run xen_stats.pl every minute
*/1 * * * * root /usr/local/bin/xen_stats.pl > /tmp/xen-stats.new && cat /tmp/xen-stats.new > /var/run/xen-stats

SNMP settings for Domain-0

Add the line below to “/etc/snmp/snmpd.conf” and then restart the snmpd service

extend xen-stats   /bin/cat /var/run/xen-stats

Cacti

I reduced Ian Cacti script to be based on a per-server setup, meaning this script gets the host (dom-0) name from Cacti, but cannot support live migrations. I will try to deal with combining live migrations with Cacti in the future.

Download and extract my modified xen_cloud.tar.gz file. Extract it, place the script and config in its relevant location, and import the template into Cacti. It should work like charm.

A note – the PHP script will work only on PHP5 and above. Works flawlessly on Centos5.2 for me.

I’ve had a belief for quite a while now that Linux, unlike other types of systems, was unable to produce any I/O SNMP information. I only recently found out that it was partially true – all production-level distros, such as RedHat (and Centos, for that matter) were unable to produce any output for any SNMP DISKIO queries.

I had found a bugzilla entry about it, so I raise the glove in a request to any of the maintainers of an RH-compatible repositories to recompile (and maintain, of course) an alternate net-snmp package which supports diskio.

Meanwhile, I have found this blog post, which offers an alternate (and quite clumsy, yet working) solution to the disk performance measurement issue in Linux. I haven’t tried it yet, but I will, rather soon.

—Update—

I have used the script from the blog post mentioned above, and it works.

Speed could be an issue. Comparing two servers the speed differential was amazing.

Both servers are connected on the same switch as the server running the query is connected. Server1 has a P2 233MHz CPU, while Server2 has a dual 2.8GHz Xion CPU.

~$ time snmpwalk -c COMMUNITY -v2c Server1 1.3.6.1.4.1.2021.13.15 > /dev/null

real 0m0.311s
user 0m0.024s
sys 0m0.020s

~$ time snmpwalk -c COMMUNITY -v2c Server2 1.3.6.1.4.1.2021.13.15 > /dev/null

real 0m8.303s
user 0m0.044s
sys 0m0.012s

Looks like a huge difference. However, I believe it’s currently good enough for me.

I love the tool Cacti. It’s a nice graphing tool, which helps a lot in system monitoring and management.

Its logic, however, is not always obvious. The relationship between data types, host templates and graph templates is quite complex, and if you are to implement any non-default setup using Cacti, you better understand these relationships as soon as possible.

I was using it to monitor an ERP system. I have encountered in the past a problem with monitoring Linux network traffic through SNMP, since the container is a short integer, and during 5 minutes wait it gets filled several times, if this is 1Gb/s network connection we measure.

The solution I wanted to employ was to haste the system, and measure every minute (60 seconds). This isn’t trivial using either Cacti or MRTG, because they were designed for 5 minute interval.

I have found this link in Cacti’s forums, which shows how to do it, and it works fine, as long as you follow instructions by the letter. This was done on RH4 x86, using Dag Wieers’ Home-made RPM repository as the source for Cacti.

I will attempt to export the modified templates and upload them here, to save some of the dirty work required for the process to work.