Things n' Stuff Thoughts about the universe in general

The following script I install as a cron job on Amazon AWS virtual machines I deploy, to allow them to backup themselves automatically. The script uses the EC2 management utilities that are normally available on “Amazon Linux” installations (and can be easily installed on other Linux distributions) to create EBS snapshots of the current mounted root EBS volume¹.
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1. I don’t expect this script to work for instances that have an instance-stored root device, but I don’t expect to encounter these any more [↩]

Please go to Google’s Home page right now to look at the Google Doodle for Robert Moog‘s¹ 78th Birthday – its a fully functional synthesizer that you can play on and record yourself!

It works slightly better in Chrome although the latest Firefoxes can also handle the moogdoodle. I wouldn’t count on anything else being able to play this demo that uses the experimental Web Audio API for HTML 5 that looks to aiming to bring to the web, all the capabilities of a Commodore 64 SID ;-).

Things you can do (The knobs are unfortunately unmarked, so I had to guess what each knob does):

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1. Inventor of the analog synthesizer [↩]

Its called ldapvi, and its just about what you’d expect with that name :-). The author would like you to think about it as vipw for your LDAP based authentication, and in addition to a few command line option it really isn’t much more then that (UI-wise):

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Nvidia Optimus is a neat solution to the problem of power consumption vs. 3D performance in notebook computers – the computer comes with two graphics chips, an “integrated graphics package” – the Intel GMA embedded graphics and a “discrete graphics package” – the Nvidia chip. The setup works by running your normal windowing UI on the integrated graphics, only powering on the discrete graphics when you want to play a 3D game or something like that.

Recently I had the fortune to work on a Lenovo T420 laptop¹ that has this setup, and it works very well on MS-Windows with the Nvidia Optimus driver – you get battry life around 10 hours with the 9 cell battery.

Unfortunately I spend most of my day in Linux and because Nvidia does not support Optimus on Linux, you have the poor choice of either running everything on the Nvidia chip – decreasing battery life to around 4 hours if you’re careful, or disabling the Nvidia chip completely.

The solution comes from the Bumblebee project – a software suite to handle the switching between the Nvidia discrete graphics and the embedded Intel chip.

The setup is pretty simple to understand (though I suspect under the hood there are many problems to be solved): A service runs and waits for users to ask for 3D accelerated graphics. When a user starts a program using the special command optirun, the service loads the Nvidia driver, starts an X server using the discrete graphics (with the display disconnected from the actual screen) and runs the specified program on that “background” X server. Then it copies the visuals from the program that is rendered using the discrete graphics to a window on the main X server. When the program terminates, the service closes the secondary X server, removes the driver and powers down the graphics card – putting us back into the ~10 hours battery life.

The Bumblebee software had some problems in the past, but the current version – 3.0 – looks very good. There are a few seconds of delay when you launch the application (setting up the driver and X takes some time), but performance is about what you’d expect when running directly on the hardware. All this without any configuration – that is if you are running on the stable Ubuntu version.

As I can’t leave well enough alone, and whenever someone says “alpha”, I say – “I wants”, I’m running the current Ubuntu 12.04 alpha (which is not so bad – due to be released in a couple of months, it works very well). And of course Bumblebee doesn’t work properly here.

So this is what I had to do to get it running:

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1. with an amazing 1600×900 screen, what they call “HD+”. Truly a work of art [↩]

In my company we (still) use an Active Directory domain controller to manage central authentication¹, which is not set up very well – no SSL and the Kreberos setup is not done properly. This makes gives much trouble to modern Linuxes (e.g. not Ubuntu. yes – I’m looking at you Shuttleworth.)  such as Fedora, as Fedora have done away with NSS/PAM based LDAP authentication and instead relies on SSS – which I have yet managed to get working or even find a tutorial on how to set it up properly.

So if you still want to authentication your Fedora installation against the company’s Active Directory – and can’t/won’t rely on Winbind’s notably flaky behaviour, you can always install NSS/PAM ldap authentication manually. Unfortunately its not as easy as it sounds, and as I learned the hard way – one must pay careful attention to SELinux. So here’s the recipe:

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1. there are a lot of MS-Windows workstation, so it kinds of makes sense – but we are planning to phase it out in favour of OpenLDAP, so don’t worry about it [↩]

When you want to host a Subversion repository on your CentOS (or RHEL) server, its quite annoying that the only options available to serve your repository are SSH (using the svn+ssh:// schema in the Subversion URLs – it is basically set up out of the box, not tweaking necessary) and Apache’s mod_dav_svn (using the http:// or https:// schema in the Subversion URLs – this is somewhat complex to setup but good instructions are easy to find on the web).

The main advantage of either of these methods is security: both have easy transport security (SSH by default, Apache if you set it to server over SSL) and are easy to setup authentication for (SSH authenticate against the system’s accounts using PAM and Apache authenticate against basically anything with a simple setup).

The main disadvantage of these methods are that they are slow (SSH is apparently somewhat faster then HTTP) and when supporting multiple large projects of many developers I started running into all kinds of weird connection errors when you try to manipulate many files on many projects at the same time.

Subversion itself offers another alternative using their own network service called svnserve – this is a standard unix daemon that listens on a specific port¹ and uses a native protocol to communicate with Subversion clients (using the svn:// schema in Subversion URLs). It offers very good performance, but no transport security (encryption) by default. Another major problem with using svnserve as a network service is that while CentOS ships the binary itself (it is required as part of the way that the svn+ssh:// protocol is implemented) it doesn’t ship any support files to run it as a standalone service nor to help with its configuration. Also by default svnserve can only authenticate users using its own Apache-style password database file – which makes it unsuitable to integrate in large organizations.

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1. port 3690 by default [↩]

This piece of script came in handy when I wrote a utility that “recycles” space on a logging partition: before log rotation archives the current log file, we move some old log files (depending on some archive freshness policy) to a remote storage that archives older files.

The problem is that the “old archive storage” also has limited disk space and I got fed up managing the archive by hand. The solution I came up is to scan the hierarchy of  log files in the storage (logs are stored hierarchically according to origin and type) and delete old files until I have enough room to move some newer files in. That way the “old archive storage” is always kept full and keeps as much back-log as possible and does this automatically.

The piece of code that determines which files we want to delete works like this:

1. Use find to list all the files in the directory structure
2. Pipe it to perl and collect all the file names in a list
3. Use perl’s sort operator to compare the modification times of each file in the list and show them in the order (i.e. oldest first)
4. Use head to get just the first file

So it looks like this:

find /mnt/httpd_back/ -type f | perl -nle 'next unless -f; push @files, $_; END { foreach $file (sort { @a=stat($a); @b=stat($b); $a[9] <=> $b[9] } @files) { print $file; }}' | head -n1


Note: normally we use head to get some initial output and terminate the process early before it does more costly work – when head has enough data it terminates the pipe sending SIGPIPE to the upstream process and that usually terminates the process that generates the data. In this case – and in all other cases involving sort – the upstream process buffers all the data in its own memory before outputting anything, so it can sort everything, and using head here is just a filter to get what I want and does not actually save me from doing all the work. I could have easily done the same thing inside the perl script itself by replacing the block of  print $file; with print $file; last; – this has the same effect as using head, because head will send SIGPIPE to perl after getting the first print and will terminate it. Deciding which way you want to go is probably more about readability of the code and I prefer my original version because its easier to read to non-perl specialists.

I can then just remove that file, see if I have enough room to move in the newer log file and if no – repeat the process.

This would work well, I believe, but it may be inefficient if I find a bunch of small files and I want to copy in a large file. So what I did next is to take advantage of the fact that all the log files I have are named using the following simple format:

<service>-<type>_log-<year><month><day>.gz

and that allows me to easily find all the log files that record the same day and eliminate them at the same time. Subsequent moving of additional files will likely succeed because I cleared out all the log files of an entire day. If not, I can always go and clear up another day’s worth of logs.

Update: The latest update for CyanogenMod for Milestone – version 7.1.0.3 solves this problem as well.

Lately I’ve been having problems updating software on my Motorola Milestone (1, as in A853) running CyanogenMod 7.1 (thanks to Nadalbak who maintains an unofficial and unsupported port of CyanogenMod for this old device). When I start the Android Market and do an update of an existing application, I get the error “Package file is invalid” for any application. Sometimes removing an application and reinstalling it will work, but often not.

There could be several problems that cause this, apparently a problem with the file permissions on the file systems is common, and if this is your problem then it can be fixed by opening the terminal emulator, executing “su” to gain super-user permissions and then running “fix_permissions” (it will take a while to complete).

If this doesn’t solve the problem for you, then likely the problem is that you ran out of space on your “data” partition or “cache” partition. This is very common if you have a large “apps partition” on your SD card and you are in the habit of installing tons of applications – I know I am 🙂 .

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As most Linux users are aware, you can access network shares with your file browser of choice (Dolphin, Nautilus and probably others) by writing URLs into the file browser’s “location bar”¹.

If you ever had to work with MS-Windows file server, you should know that you can also access SMB file shares using URLs – with the schema “smb”, like this: smb://file-server/share-name.

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1. and this works just as well with the desktop’s “run dialog” – try holding ALT-F2 and typing something like ftp://ftp.mirrorservice.org/ and up comes your file browser showing the FTP site’s directories. This won’t work for GNOME 3, at least at this time. [↩]

This is a short “how to” article on how to get and install the new Microsoft Consolas Monotype font – this is a very nice programmer font and as described in Dan Benjamin’s “Top 10 Programming Fonts” it is highly recommended for use in your programmer’s editor or IDE (As a side note, his top selection, Inconsolata is also very nice and I highly recommend it if you don’t feel like using Microsoft created software and/or aren’t interested in performing the steps below which may or may not be legally problematic).

The main problem with using Consolas on your Linux workstation, is that this font is provided by Microsoft, and while if you are running MS-Windows (or even Mac OS-X) it may already be installed – as it is bundled with many Microsoft products, us in the Linux world have no easy (and legal) way to get to use Consolas in our code editing. Unlike Microsoft Core Fonts for the Web Consolas is not available for download – probably for the same reason the core fonts packages were pulled. But as Consolas is bundled with many Microsoft products, some are available for free download for anyone, we can use that to get us some nifty Consolas prettiness in our day to day Linux computing.

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