Logical Volume Manager Cheatsheet

Martin Ankerl — Mon, 04 Feb 2008 23:08:47 +0000

Today I got a new harddisk, so it was finally time for me to have an in depth look at LVM. I use it to combine two harddisks /dev/sdg1 and /dev/sdb1 into one volume group ext_vg which contains one big logical volume /dev/ext_vg/ext. In short, my 250GB and 500GB harddisks are used in one big 750GB ext3 mount point.

I followed the excellent LVM Tutorial, and was surprised how easy everything goes. Here is a short cheat sheet of the important commands (take care to exchange the partition/volume names if you use this):

Installation

I use Ubuntu, so this is a piece of cake:

sudo apt-get install lvm2

There is also a GUI available, but I found the command line tools are very easy to use so there is no real need. I wouldn’t use EVMS as it is not supported in Ubuntu 7.10 and may cause problems.

Create a New Filesystem

First I create the physical volume on the partition sdg1, create a new volume group ext_vg that contains this physical volume, and create a new logical volume of size 450GB within the volume group. Finally create the filesystem (disabled reservation space, see Get More Space Out of Your ext3 Partition).

sudo pvcreate /dev/sdg1
sudo vgcreate ext_vg /dev/sdg1
sudo lvcreate -L 450G -n ext ext_vg
sudo mkfs.ext3 -m 0 /dev/ext_vg/ext

Show Status

Each LVM layer has its corresponding command to get information about the metadata:

sudo pvdisplay
sudo vgdisplay
sudo lvdisplay
sudo pvs
sudo vgs
sudo lvs

Mount via fstab

I use the filesystem mainly for data, so allowing just rw is enough (no executables allowed). noatime allows quicker access.

sudo mkdir /media/mega
/dev/ext_vg/ext /media/mega     ext3    rw,noatime,user 0       2
mount /media/mega

Resize

It is even possible to do an online resize of the system, wohoo This extends the logical volume by 200MB.

sudo lvextend -L +200 /dev/ext_vg/ext
sudo resize2fs -p /dev/ext_vg/ext

You can watch the resize process going on with df -h.

Add Another Partition to the Logical Volume

To add another partition and use up all the available space in the logical volume, first add the physical volume to the volume group, then use pvdisplay to find out the total available number of free PE (add the numbers from the physical volumes), then use lvextend to use up all this available space.

sudo pvcreate /dev/sdb1
sudo vgextend ext_vg /dev/sdb1
sudo pvdisplay
sudo lvextend -l +63602 /dev/ext_vg/ext
sudo resize2fs /dev/ext_vg/ext

That’s it! Any questions? please post.

Ajax Dojo Comet Tutorial

Martin Ankerl — Tue, 21 Aug 2007 12:41:22 +0000

EDIT: This tutorial is for an old version of dojo / comet, and it will not work in a recent version!

Markus Holzmann, an intern at Profactor of my fellow colleague Philipp Hartl, had the opportunity to experiment with Ajax during his job. He wrote a tutorial about how to push events from the server to the client. For example, display popup messages on all browsers at the same time (see screencast in full resolution here):

Read on how Markus did this:

Cometd Hello World

I’ve read Chris Bucchere’s Say Hello World to Comet and built an application based on this using a more current version of Jetty (version 6.1.5) which I embedded into a Tomcat v5.5 Server. For the developing I used Eclipse 3.2.

Start your Engines

At first you have to get the server running. As I mentioned I embedded Jetty into a Tomcat server. Therefore you have configure the libraries:

Add the packages org.mortbay.cometd and dojox.cometd to your source folder and delete the client package in the org.mortbay.cometd package.
Add jetty-util-6.1.5.jar, jetty-6.1.5.jar and servlet-api-2.5-6.1.5.jar to your build path.
Copy the jetty-util-6.1.5.jar file into the /lib folder in the WEB-INF directory.

Replace the existing servlets in your web.xml – file in the WEB-INF – folder with the following servlets:


  cometd
  org.mortbay.cometd.continuation.ContinuationCometdServlet
  1


  cometd
  /cometd/*

For the project I used the dojo toolkit (version 0.4.3) which has an integrated COMETd class that makes it easy to build comet projects. Download it and add it to your WebContent folder.

When you’ve done all this, the hardest piece of work for this program is already done.

Hack the Code

Now you can implement the code for the client side: You need a HTML file with a button on it. The code for this looks like this (download):

Line by line, the above bold code works like this:

In the line
you integrate the dojo toolkit into the project.
To activate the cometd class of dojo:
```
dojo.require("dojo.io.cometd");
```
Connect the server with the client:
```
cometd.init({}, "cometd");
```

Here we say what to do when there is a subscribe event:

cometd.subscribe("/hello/world", false, "publishHandler");

Last but not least, the publishHandler function serves as the callback function, which uses alert to show a simple message box:
```
publishHandler = function(msg) {
  alert(msg.data.test);
}
```

Give it a Try

When you load the HTML file now, you can click on the button and an alert box saying hello world will appear:

The reason for this is that when you click the code

cometd.publish('/hello/world', { test: 'hello world' } )

is executed which publishes a text on the channel with the id /hello/world.

The funny thing is that this is able to run on any number of browsers. Everytime when a client clicks the button, on all browsers that view this page the alert box is shown. (See screencast above).

Pushing Data from Server to Client

You can also add serverside code to trigger an event. I wrote a JSP file with the following code:

<%@page import="java.util.*"%>
<%@page import="dojox.cometd.*" %>
<%
Bayeux b = (Bayeux)getServletContext().getAttribute(Bayeux.DOJOX_COMETD_BAYEUX);
Channel c = b.getChannel("/hello/world",false);

Map message = new HashMap();
message.put("test", "jsp: hello world");

c.publish(b.newClient("server_user",null),message, "new server message");
%>

When this page is loaded, an alert popup appears at the page saying jsp: hello world.

That’s it. Happy hacking!

Martin Ankerl » tutorial