Archive for June, 2007

VMware Fencing in RedHat Cluster 5 (RHCS5)

Thursday, June 14th, 2007

Cluster fencing – Unlike many common thoughts, high-availability is not the highest priority of an high-availability cluster, but only the 2nd one. The highest priority of an high-availability cluster is maintenance of data integrity by prevention of multiple concurrent access of nodes to the shared disk.

On different cluster, depending on the vendor, this can be achieved by different methods, either by prevention of access based on the status of the cluster (for example – Microsoft Cluster, which will not allow access to the disks without cluster management and coordination), by panicking the node in question (Oracle RAC, for example, or IBM HACMP), or by preventing failover unless the status of the other node, as well as all heartbeat links were ok up to the exact moment of failure (VCS, for example).

Another method is based on a fence, or “Shoot the Other Node in the Head”. This “fence” is usually based on an hardware device which has no dependencies for the node’s OS, and is capable of shutting it down, many times brutally, upon request. A good fencing device can be a UPS, which supports the other node. The whole idea is that in a case of uncertainty, either one of the nodes can attempt to ‘kill’ the other node, independently of any connectivity issue one of them might experience. This race result is quite obvious: one node remains alive, capable of taking over the resource groups, the other node is off, unable to access the disk in an uncontrolled manner.

Linux-based clusters will not force you to use fencing of any sort, however, for a production environments, setups without any fencing device will be unsupported, as the cluster cannot handle cases of split-brain or uncertainty. These hardware devices, which can be, as said before, a manageable UPS, a remote-control power-switch, the server’s own IPMI (or any other independent system such as HP ILO, IBM HMC, etc), and even the fiber switch – as long as it can prevent the node in question from accessing the disks, are quite expensive, but comparing to hours of restore-from-backup, they sure justify their price.

On many sites there is a demand for a “test” setup which will be as similar to the production setup as possible. This test setup can be used to test upgrades, configuration changes, etc. Using fencing in this environment is important, for two reasons:

1. Simulation of the production system behavior is achieved with as similar setup as possible, and fencing takes an important part in the cluster and its logic.

2. A replicated production environment contain data which might have some importance, and if not that, at least re-replicating it from the production environment after a case of uncontrolled access to the disk by a faulty node (and this test cluster is in a higher risk, as defined by its role), or restoring from tapes is unpleasant and time consuming.

So we agree that the test cluster should have some sort of fencing device, even if not similar to production’s one, for the sake of the cluster logic.

On some sites, there is a demand for more than one test environment. Both setups – a single test environment and multiple test environments can be defined to work as guests on a virtual server. Virtualization assists in saving hardware (and power, and cooling) costs, and allows for easy duplication and replication, so this is a case where it is ideal for the task. This said, it brings up a problem – fencing a virtual server has implications – we can kill all guest systems in one go. We wouldn’t want that to happen. Lucky for us, RedHat Cluster has a fencing device for VMware, which, although not recommended in a production environment, will suffice for a test environment. These are the steps required to setup one such VMware fencing device in RHCS5:

1. Download the latest CVS fence_vmware from here. You can use this direct link (use with “save target as”). Save it in your /sbin directory under the name fence_vmware, and give it execution permissions.

2. Edit fence_vmware. In line 249 change the string “port” to “vmname”.

3. Install VMware Perl API on both cluster nodes. You will need to have gcc and openssl-devel installed on your system to be able to do so.

4. Change your fencing based on this example:

<?xml version="1.0"?>
<cluster alias="Gfs-test" config_version="39" name="Gfs-test">
        <fence_daemon clean_start="0" post_fail_delay="0" post_join_delay="3"/>
                <clusternode name="cent2" nodeid="1" votes="1">
                                <method name="1">
                                        <device name="man2"/>
                <clusternode name="cent1" nodeid="2" votes="1">
                                <method name="1">
                                        <device name="man1"/>
                                <method name="2">
                                        <device domain="22 " name="11 "/>
        <cman expected_votes="1" two_node="1"/>
                <fencedevice agent="fence_vmware" name="man2"
                          ipaddr="" login="user" passwd="password"
                <fencedevice agent="fence_vmware" name="man1"
                          ipaddr="" login="user" passwd="password"
                        <fs device="/dev/sda" force_fsck="0" force_unmount="0"
				fsid="5" fstype="ext3" mountpoint="/data"
                                name="sda" options="" self_fence="0"/>
                <service autostart="1" name="smartd">
                        <ip address="" monitor_link="1"/>
                <service autostart="1" name="disk1">
                        <fs ref="sda"/>

Change to your relevant VMware username and password.

If you have a Centos system, you will be required to perform these three steps:

1. ‘ln -s /usr/sbin/cman_tool /sbin/cman_tool

2. ‘cp /etc/redhat-release /etc/redhat-release.orig

3. ‘echo “Red Hat Enterprise Linux Server release 5 (Tikanga)” > /etc/redhat-release

This should do the trick. Good luck, and thanks again to Yoni who brought and fought the configuration steps.


Per comments (and a bit-late – common logic) I have broken lines in the XML quote for cluster.conf. In cases these line breaks might break something in RedHat Cluster, I have added the original xml file here: cluster.conf

More on the Nabaztag/tag

Wednesday, June 13th, 2007

Actually, this post has become less of the non-technical type and more of the technical type, however, for the sake of the cute little Nabaztag (you can send me messages too! Go here and send a message to “fatutchi”!), I keep it still in this category as well.

Today is a busy day, so I’ll have several posts.

This one will deal with the Nabaztag/tag. I have extended the PHP form/script offered in my previous post to allow for multiple Nabaztags selection. Also, added reading the ears status, and parsing the XML returned by the Nabaztag API site.

This is an ugly script, but it works. As said before – if you see fit to extend it or add features, please do so. Attached here: nabiV2.php.txt

I have noticed Violet had several issues with their site. I must confess that I have expected more from their site. As I’ve been involved as a consultant in several large-scale setups which sustained several tenth of thousands (and more) of connections per second, I know that, usually, the main performance hog is caused by an inefficient application design. It could be that Violet’s problems might just point at a low quality server-side software. Pity. I hope it will get better.

I have a Nabaztag/tag

Tuesday, June 12th, 2007

I have received my Nabaztag/tag just a day ago, and it is a cute little thingie.


What can it do?

Actually, not much. It is a wireless device (client) which access Violet’s Nabaztag servers to get its commands. You, theoretically, cannot hijack the session and use it directly over LAN, but you must go through the Internet. This leads to delays in assigning commands.

It can move its ears (surprisingly, very quietly), it can play sound, either by text-to-voice (probably happens on the server-side) or streaming MP3 (cannot, as far as I’ve noticed, play MMS directly). It can also report its ears positions.

I can fly!

As you probably know, the more important thing is not about what it can do, but about how we can utilize it. Violet has added a list of RSS sources for the Nabaztag to read aloud. Through server-side sub processes, it can tell the time (usually at full hours), it can act as a wakeup clock (doesn’t do its job for me – not enough to wake me up), etc. It can probably take part in games based on the location of the ears (for example, if you agree, move the right ear down, etc).

You can check wikipedia for its entry. They cover most of it, maybe except for how cute it is, and it is.

If it were to end at this, I would have been quite frustrated, especially with the device’s price. However, Violet has exported an API which allows me (and you, and him, and everyone!) to send commands (unlimited by the number, as it seems) to the Nabaztag – Say this, move your ears to this position, etc. It allows me to send a choreography, aka a dance, to the device, and it will perform it based on the timing set by the sender.

I have wasted some of my day yesterday to write down a simple (and quite ugly, if you ask me) form which will use the API for simple commands. In my TODO list there is to implement the whole choreography thing, and make it easy. I would like to build this interface as a base for possible other utilizations, such as community games, etc.

I have uploaded my API using PHP form here which is free for use (of course) and everyone is encouraged to use it and/or modify it, as long as you give me my credits :-). I’m not sure about its security yet… nabi.php.txt

It’s a raw thing, but it works. Don’t forget to:

1. Activate your API interface in

2. Change the parameters of your SN and your TOKEN

3. Place the script on PHP enabled site.


Linux LVM performace measurement

Sunday, June 10th, 2007

Modern Linux LVM offers great abilities to maintain snapshots of existing logical volumes. Unlike NetApp “Write Anywhere File Layout” (WAFL), Linux LVM uses “Copy-on-Write” (COW) to allow snapshots. The process, in general, can be described in this pdf document.

I have issues several small tests, just to get real-life estimations of what is the actual performance impact such COW method can cause.

Server details:

1. CPU: 2x Xion 2.8GHz

2. Disks: /dev/sda – system disk. Did not touch it; /dev/sdb – used for the LVM; /dev/sdc – used for the LVM

3. Mount: LV is mounted (and remains mounted) on /vmware


1. No snapshot, Using VG on /dev/sdb only:

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 0m16.088s
user 0m0.009s
sys 0m8.756s

2. With snapshot on the same disk (/dev/sdb):

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 6m5.185s
user 0m0.008s
sys 0m11.754s

3. With snapshot on 2nd disk (/dev/sdc):

# time dd if=/dev/zero of=/vmware/test.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 5m17.604s
user 0m0.004s
sys 0m11.265s

4. Same as before, creating a new empty file on the disk:

# time dd if=/dev/zero of=/vmware/test2.2GB bs=1M count=2048
2048+0 records in
2048+0 records out

real 3m24.804s
user 0m0.006s
sys 0m11.907s

5. Removed the snapshot. Created a 3rd file:

net-snmp broken in RHEL (and Centos, of course) – diskio

Saturday, June 9th, 2007

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.


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 > /dev/null

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

~$ time snmpwalk -c COMMUNITY -v2c Server2 > /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.