Tech Blog

Learnosity are looking for a DevOps/Linux Engineer to join our growing team.

Note: this role has now been filled. Thanks to all who applied.

Learnosity is an award winning company which develops cutting edge tools for language learning and is used by the leading educational publishers and assessment companies globally. We deliver millions of assessments every year to users on 6 continents.

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The message they got was:

I knew that the site and file were both available and a bit of hunting around I was reminded that IE used to have issues with Gzip encoding in the past - but I presumed that it was a thing of the past.

I also noticed that when downloading the file Firefox would not show a progress bar but would instead show a "indeterminate progress" bar.

Looking at the headers the it turned out that there was no Content-Length header but there was a header as follows:

To fix this I added some more exclusions to my mod_gzip rules to not zip some particular file types.

Now its working a treat. It would also be possible to do this only for IE - but most of the files we use on this are already well compressed to gzip compression doesn't help too much anyway.

Cheers, Mark

Change the "tomcat" to any other unique identifier to pick a different process.

Thanks to Maximdim for his post on this.

The JStat reference has the low down on what all the codes mean.

Block an IP:

Unblock an IP (if you typed the wrong address)

Show current rules:

Thanks to good folks at www.pbxer.com for the details on this:Asterisk Security: Use iptables to Block the Bad Guys

Initial attempts using ffmpeg were making it re-encode the entire video which would take ages and result in a larger file or worse quality.

A bit of googling and reading the man pages later I discovered

This tells ffmpeg to copy the video and audio without re-encoding.

So insted of this

The Solution

Further complications

This worked a treat for all except one file - which gave the following error.

By using the "-an" and "-vn" flags to skip the video and audio encoding in turn I narrowed it down to a problem in the audio codec timestream.

To try to get ffmpeg to fix the problem I got it to reencode the audio but copy the video codec with the following:

Worked a treat. I love ffmpeg :-)

Update

Final code to fix it using aac with audio quality quite high 200 (range is 0-255)

• Makes it practical for students to practice Oral and Aural skills
• Is efficient and effective for teachers, as they can listen to each student individually at a time to suit them
• Can be used for homework assignments or “High Stakes Assessments”

Learnosity are creating the next generation of language and assessment technology for use in schools and education worldwide. The current product portfolio includes:

• High availability web based systems
• Cross platform software (Windows, Mac and Linux)
• iPhone/Android native applications
• Telephony/VOIP and SMS applications

We need someone who can:

• Use Javascript or Actionscript to create great user interfaces
• Develop highly scalable web applications using ColdFusion, PHP or similar
• Cut code with the best in the world

You will also need to be:

• keen to continue learning new technologies
• able to have a conversation with non technical people
• enthusiastic and ready to push the boundaries

You'll need:

• 1-3 years of programming experience
• Experience in at least one Client side language (Actionscript or Javascript/jQuery)
• Experience in at least one Server side language (eg PHP, Java, ColdFusion, etc)
• Understanding of Object Oriented design
• Understanding of XHTML and CSS

It would be good if you have:

• A degree in Computer Science, Engineering or similar.
• been working with open source tools
• Experience with Adobe AIR or Flex
• been playing around with iPhone/Android applications
• experience with some of Linux/VOIP/SIP/Asterisk/Jabber/XMPP

This is a full time role and you will be working in a casual workplace with flexible hours in the Sydney CBD. Salary commensurate with experience.

If this sounds like the job for you, email a covering letter explaining why you'll be great and your resume to mark@learnosity.com - no agencies please.

If you copy code from on svn codebase to another then you can often have hidden .svn directories copied also, which will cause svn operations to fail.

Running the command above will fix it up by finding them all and deleting them.

This guide explains how to set up software RAID1 on an already running Debian Etch system. The GRUB bootloader will be configured in such a way that the system will still be able to boot if one of the hard drives fails (no matter which one).

I do not issue any guarantee that this will work for you!

1 Preliminary Note

In this tutorial I'm using a Debian Etch system with two hard drives, /dev/sda and /dev/sdb which are identical in size. /dev/sdb is currently unused, and /dev/sda has the following partitions:

In the end I want to have the following situation:

This is the current situation:

2 Installing mdadm

The most important tool for setting up RAID is mdadm. Let's install it like this:

apt-get install initramfs-tools mdadm

You will be asked the following question:

MD arrays needed for the root filesystem: <-- all

Afterwards, we load a few kernel modules (to avoid a reboot):

Now run

The output should look as follows:

3 Preparing /dev/sdb

To create a RAID1 array on our already running system, we must prepare the /dev/sdb hard drive for RAID1, then copy the contents of our /dev/sda hard drive to it, and finally add /dev/sda to the RAID1 array.

First, we copy the partition table from /dev/sda to /dev/sdb so that both disks have exactly the same layout:

The output should be as follows:

The command

Next we must change the partition type of our three partitions on /dev/sdb to Linux raid autodetect:

To make sure that there are no remains from previous RAID installations on /dev/sdb, we run the following commands:

If there are no remains from previous RAID installations, each of the above commands will throw an error like this one (which is nothing to worry about):

Otherwise the commands will not display anything at all.

4 Creating Our RAID Arrays

Now let's create our RAID arrays /dev/md0, /dev/md1, and /dev/md2. /dev/sdb1 will be added to /dev/md0, /dev/sdb2 to /dev/md1, and /dev/sdb3 to /dev/md2. /dev/sda1, /dev/sda2, and /dev/sda3 can't be added right now (because the system is currently running on them), therefore we use the placeholder missing in the following three commands:

The command

should now show that you have three degraded RAID arrays ([_U] or [U_] means that an array is degraded while [UU] means that the array is ok):

Next we create filesystems on our RAID arrays (ext3 on /dev/md0 and /dev/md2 and swap on /dev/md1):

Next we must adjust /etc/mdadm/mdadm.conf (which doesn't contain any information about our new RAID arrays yet) to the new situation:

Display the contents of the file:

At the bottom of the file you should now see details about our three (degraded) RAID arrays:

5 Adjusting The System To RAID1

Now let's mount /dev/md0 and /dev/md2 (we don't need to mount the swap array /dev/md1):

You should now find both arrays in the output of mount

Next we modify /etc/fstab. Replace /dev/sda1 with /dev/md0, /dev/sda2 with /dev/md1, and /dev/sda3 with /dev/md2 so that the file looks as follows:

Next replace /dev/sda1 with /dev/md0 and /dev/sda3 with /dev/md2 in /etc/mtab:

Now up to the GRUB boot loader. Open /boot/grub/menu.lst and add fallback 1 right after default 0:

This makes that if the first kernel (counting starts with 0, so the first kernel is 0) fails to boot, kernel #2 will be booted.

In the same file, go to the bottom where you should find some kernel stanzas. Copy the first of them and paste the stanza before the first existing stanza; replace root=/dev/sda3 with root=/dev/md2 and root (hd0,0) with root (hd1,0):

root (hd1,0) refers to /dev/sdb which is already part of our RAID arrays. We will reboot the system in a few moments; the system will then try to boot from our (still degraded) RAID arrays; if it fails, it will boot from /dev/sda (-> fallback 1).

Next we adjust our ramdisk to the new situation:

Now we copy the contents of /dev/sda1 and /dev/sda3 to /dev/md0 and /dev/md2 (which are mounted on /mnt/md0 and /mnt/md2):

6 Preparing GRUB (Part 1)

Afterwards we must install the GRUB bootloader on the second hard drive /dev/sdb:

On the GRUB shell, type in the following commands:

Now, back on the normal shell, we reboot the system and hope that it boots ok from our RAID arrays:

7 Preparing /dev/sda

If all goes well, you should now find /dev/md0 and /dev/md2 in the output of

The output of cat /proc/mdstat should be as follows:

Now we must change the partition types of our three partitions on /dev/sda to Linux raid autodetect as well:

Now we can add /dev/sda1, /dev/sda2, and /dev/sda3 to the respective RAID arrays:

Now take a look at cat /proc/mdstat ... and you should see that the RAID arrays are being synchronized:

(You can run watch cat /proc/mdstat to get an ongoing output of the process. To leave watch, press CTRL+C.)

Wait until the synchronization has finished (the output should then look like this:

Then adjust /etc/mdadm/mdadm.conf to the new situation:

/etc/mdadm/mdadm.conf should now look something like this:

8 Preparing GRUB (Part 2)

We are almost done now. Now we must modify /boot/grub/menu.lst again. Right now it is configured to boot from /dev/sdb (hd1,0). Of course, we still want the system to be able to boot in case /dev/sdb fails. Therefore we copy the first kernel stanza (which contains hd1), paste it below and replace hd1 with hd0. Furthermore we comment out all other kernel stanzas so that it looks as follows:

In the same file, there's a kopt line; replace /dev/sda3 with /dev/md2 (don't remove the # at the beginning of the line!):

Afterwards, update your ramdisk:

... and reboot the system:

It should boot without problems.

That's it - you've successfully set up software RAID1 on your running Debian Etch system!

9 Testing

Now let's simulate a hard drive failure. It doesn't matter if you select /dev/sda or /dev/sdb here. In this example I assume that /dev/sdb has failed.

To simulate the hard drive failure, you can either shut down the system and remove /dev/sdb from the system, or you (soft-)remove it like this:

Shut down the system:

Then put in a new /dev/sdb drive (if you simulate a failure of /dev/sda, you should now put /dev/sdb in /dev/sda's place and connect the new HDD as /dev/sdb!) and boot the system. It should still start without problems.

Now run

cat /proc/mdstat

and you should see that we have a degraded array:

The output of

fdisk -l

should look as follows:

Now we copy the partition table of /dev/sda to /dev/sdb:

(If you get an error, you can try the --force option:

sfdisk -d /dev/sda | sfdisk --force /dev/sdb

)

Afterwards we remove any remains of a previous RAID array from /dev/sdb...

... and add /dev/sdb to the RAID array:

Now take a look at

Wait until the synchronization has finished:

Then run

and install the bootloader on both HDDs:

That's it. You've just replaced a failed hard drive in your RAID1 array.

10 Links

• The Software-RAID Howto: http://tldp.org/HOWTO/Software-RAID-HOWTO.html
• Debian: http://www.debian.org

It was all relatively painless and it's quite amazing how far Wine has come.

Installing Lego Digital Designer

Go to the Lego Digital Designer site and download the Windows version.

I got LDD 3.1.3 which was the latest version available at the time.

Now - to make it all work you need to install Wine and I used the latest version from the wine PPA team.

Add the wine ppa:

You'll then see:

Now to update the list of available software:

And then install wine and winetricks

Wine lets you run Windows software on other operating systems, in this case Ubuntu.

Wine tricks provides a simple way to install native versions of some of the libraries that are not 100% compatible - by looking at the Wine App DB I discovered that it had problems with missing font's (Tahoma) and scrolling which was fixed by installing Quartz.

Then install double click on the install file for LDD and it should be up an running in no time.

If you get a warning about it not being Executable you may need to right click on the application and select Properties - and then from the Permissions tab select "Allow executing file as program"

Son is now very happy with Lego Digital Designer :-)

The find utility has an option to find a file newer than another file. By creating and empty file with a specific creation date we can do the search:

To show all files newer than 2010-01-01 use:

Or to create a tar archive of them use xargs like so:

Easy. Mark