Accelerating a RAID-5 array with a solid-state hard drive.

May 19 2019

A couple of weeks ago, one of my co-workers mentioned in passing that he'd surprised himself by adding an SSD (solid state drive) to his file server at home.  To recap a bit, Leandra, my primary server at home has a sizable RAID-5 array storing all of my data.  However, one of the tradeoffs is that stuff recently written to the array is a little slow to be read back.  It's really not noticeable unless you're logged in and running commands, and even then the lag is something like one or two seconds.  Noticeable but not actually problematic.  At any rate, I'd been wanting to do some tinkering lately and had an Amazon order planned because I wanted to do some electronic work on my warwalking rig so I figured that, depending on the cost, I might add an SDD to my order.  Much to my surprise, a 120 gigabyte SSD is incredibly cheap, I paid a hair under $20us for a Kingston A400.  Emminently affordable.

Linux on the Dell XPS 15 Touch (9570)

Mar 03 2019

UPDATED: 18 March 2019 - External display adapters that actually work with this model (and Arch Linux) added.

For various reasons, I found that I had a need to upgrade Windbringer's hardware very recently.  This might be the first time that a catastrophic failure of some kind was not involved, so it's kind of a weird feeling to have two laptops side by side, one in process and one to do research as snags cropped up.  This time around I bought a Dell XPS 15 Touch (9570) - I was expecting things to be substantially the same, but this did not seem to be the case.  Some things that I found myself ignoring because I had no use for them aren't in this newer model, and some things have changed as technology has advanced rather a lot in the last five years.

As before, first I'll post the hardware specs, and then follow up with everything I had to tinker with to get working as well as how I went about it.  As usual, I went with 64-bit Arch Linux (2019.02 installation build).

VLC crashes when trying to play stuff over the network from Kodi.

Dec 02 2018

This took me a while to figure out, so here's a fix for an annoying problem:

Let's say that you have a media box running Kodi on your local area network.  You have uPNP turned on so you can stream videos from your media box across your LAN.  You want to use VLC to watch stuff across your LAN.

Problem: When you select your Kodi box in VLC and double-click on the server to open the directory of media to watch, VLC crashes with no error message (even in debug mode).

Explanation: VLC is configured to exit when the current playlist is over.  This includes downloading a playlist across the network, and is really irritating.

Solution: In VLC, go to Tools -> Preferences -> Show Settings: All.  Scroll down to Playlist.  Un-check Play and Exit.  Save.


Ansible: Reboot the server and pick up where it left off.

Nov 26 2018

Here's the situation: You're using Ansible to configure a machine on your network, like a new Raspberry Pi.  Ansible has done a bunch of things to the machine and needs to reboot it - for example, when you grow a Raspbian disk image so that it takes up the entire device, it has to be rebooted to notice the change.  The question is, how do you reboot the machine, have Ansible pick up where it left off, and do it in one playbook only (instead of two or more)?

I spent the last couple of days searching for specifics and found a number of techniques that just don't work. After some experimentation, however, I pieced together a small snippet of Ansible playbook that does what I need.  Because it was such a pain to figure out I wanted to save other folks the same trouble.  Here's the code, suitable for copying and pasting into your playbook:

...the first part of your playbook goes here.
    - name: Reboot the system.
      shell: sleep 2 && shutdown -r now
      async: 1
      poll: 0
      ignore_errors: true
    - name: Reconnect and resume.
      local_action: wait_for
        host: bob-newhart
        port: 22
        state: started
        delay: 10
        timeout: 30
...the rest of your playbook goes here.

Specifics of proof of concept for later reference:

  • Ansible v2.7.0
  • Raspberry Pi 3
  • Raspbian 2018-06-27

Build your own time server with a GPS receiver.

Nov 24 2018

If you've had your ear to the ground lately, you might have heard that the NIST timekeeping radio station used by devices all over the world as a time reference for Coordinated Universal Time as well as some experiments in signal propagation and geophysical event notices might be on the chopping block in 2019, leaving the HF bands quieter and, let's face it, we can't have nice things.  Clocks that rely on this time source signal won't have any way to stay in sync and the inevitable drift due to the imperfections in everything will cause fractions of second to be lost and a fresh outbreak of kinetic pattern baldness.  The ultimate effects of this latest bit of clueless petulance on the part of Donald Trump remain to be seen, but it seems likely that this isn't a sexy enough problem to catch brainshare like Y2k did.  If you work extensively with computers chances are you're not that worried because your machines use NTP - the Network Time Protocol - to synch their internal clocks with a known time reference server on the Net someplace.  Something to consider, however, is whether or not your upstream tier-one and tier-two time sources are actually using the NIST WWV time singnals as their reference signals.  There is, however, a nifty way around this: Build your own NTP server that uses a reference time source that can't be shut off as a source, the Global Positioning System.

First, I'll show you how to build your own GPS time server, and then I'll explain why it works.

Life and times.

Oct 14 2018

Long time readers are probably wondering where I've been lately.  The answer is kind of long and is worth a post all on its own.  The short version of the story is, work's been eating me alive lately.  This is our busiest time of year and it's been all hands on deck for a couple of weeks now.  In point of fact, last week was our quarterly all-hands meeting, where everybody on my team was flown into town for a solid week of meetings.  All day, every day.  Most of my visible activity lately took the form of parts of my exocortex running on automatic with some hit-and-run posting while waiting for the coffee maker at work to top me up in between meetings.

This also means that I haven't had a whole lot of patience for interacting with people.  Not in the sense that people can feel frustrated with other people or their actions, but in the sense that interacting with people in a meaningful way - having a real conversation - takes more compute cycles than I have available right now.  After fourteen hours in a conference room with 40 other people, not only am I out of social, but I'm mentally exhausted.

Interfacing Huginn with Mastodon.

Aug 18 2018

It seems that there is another influx of refugees from a certain social network that's turned into a never ending flood of bile, vitriol, and cortisol into what we call the Fediverse, a network of a couple of thousand websites running a number of different applications that communicate with each other over a protocol called ActivityPub.  Ultimately, the Fediverse is different from Twitter and Facebook in that it's not run as a for-profit entity. There are no analytics, no suggestions of "thought leaders" you might want to follow, no automated curation of the posts you can see versus the ones you really want to see.  Socially speaking, you don't find people carefully polishing their brands or trying to game hashtag trends but instead everything from somebody kicking back after work with a cup of coffee to people carefully archiving the firmware of classic computer hardware to in-jokes about pineapples.  Rather than fame, you get people.

But that's not what I want to talk about.  I've been asked by a couple of people to post a brief tutorial of how I interfaced my Huginn instance with, the Mastodon instance that I spend most of my time hanging out on.

The Doctor's boot care regimen.

Jul 14 2018

Boots: 14 hole Doc Martens, black, real leather.


Wipe down with damp paper towels.

Wipe down with dry paper towels.

Coat with Dr. Martens Wonder Balsam using included sponge.  Be sure to work balsam into stitches and exposed edges.  I ordinarily don't like to shill for particular products, but I started using this stuff to help break in my boots (it makes the leather softer, so it adapts to your feet more readily) and I was wearing them clubbing within a month of getting them (instead of six months to a year).  It's amazing stuff.

Wait half an hour.  Get some coffee, go for a run, something like that.

Buff balsam off with a clean, dry cloth.  I use a regular washcloth set aside for doing my boots.

Prep your boot polish.  I like Kiwi Shoe Polish Paste, just make sure it's the right color for your boots.  Pop the lid and set the polish on fire with a lighter or matches.  No, seriously, I mean set it on fire.  The polish will melt faster than it burns.  When at least half the polish is burning, drop the lid back on and make sure it closes completely.

Wait.  The flame will burn itself out because the oxygen inside the container (there isn't much) will be used up.  Wait for the pressure to build up inside the tin and pop the lid off with a festive "Poing!"

(If this doesn't happen inside of five minutes, just open the tin.  No big deal.)

The shoe polish is now a thick goop instead of a waxy mass.  Apply polish to your boots with a sponge.

Wait another half hour.

Buff dried polish off with a clean, dry cloth.  I usually flip the washcloth over and use that, but do whatever works.  Rub until the finish doesn't look smoky anymore.  Mine tend to look clean but a little on the dull side.  That goes away as I wear them for a while.

Re-lace and wear for an hour or two to take advantage of the new dose of balsam soaked into the leather making it a bit softer than usual.

Repeat every one or two months, or after cleaning them if they get dirty.

Setting random backgrounds in LXDE.

Jun 28 2018

So, here's the situation:

On Windbringer, I habitually run LXDE as my desktop environment because it's lightweight and does what I need: It manages windows, gives me a menu, and stays out of my way so I can do interesting things.  For years I've been using a utility called GKrellm to implement not only system monitoring on my desktop (because I like to know what's going on), but to set and change my desktop background every 24 hours.  However, GKrellm has gotten somewhat long in the tooth and I've started using something different for realtime monitoring (but that's not the point of this post).  So, the question is, how do I set my background now?  Conky doesn't have that capability.

I tried a few of the old standbys like feh and nitrogen, but they didn't seem to work.  The reason for this appears to be that PCmanFM, which is both the file manager and the desktop... stuff... of LXDE.  By this, I refer to the desktop icons as well as the background image.  As it turns out, nothing I tried to change the background worked, and that is due to the fact that PCmanFM is a jealous desktop module and doesn't let other tools frob the settings it's in charge of.  After some tinkering, here's how I did it:

Short form: pcmanfm -w `ls -d -1 /home/drwho/backgrounds/* | shuf -n 1`

Long form (from inside to outside):

  • ls -d -1 /home/drwho/backgrounds/* - List all of the files in /home/drwho/backgrounds.  Show the full path to each file.  List everything in a single column.
  • | - Feed the output of the last command to the input of the next command.
  • shuf -n 1 - shuf is a little-known GNU Coreutils tool which randomly shuffles whatever things you give it.  It only returns one line of output, a randomly chosen image file.
  • The output of the previous two commands (captured between back-ticks) is passed to...
  • pcmanfm -w - Set the current desktop background to whatever filename is passed on the command line as a free action.

To set an initial background when I log in, I added the following command to my ~/.config/lxsession/LXDE/autostart file: @pcmanfm -w `ls -d -1 /home/drwho/backgrounds/* | shuf -n 1`

This means that the command will run every time my desktop starts up.  The @ symbol tells lxsession to re-run the command if it ever crashes.  However, how do I change my background periodically?

The easiest way to set that up was to set a cron job that runs every day.  Every user gets their own set of cron jobs (called a crontab) so you don't need any particular privileges to do this (unless your machine's really locked down).  If you've never set a cronjob before, the command I used was this: crontab -e

My cronjob looks like this: 00 10 * * * pcmanfm -w `ls -d -1 /home/drwho/backgrounds/* | shuf -n 1`

"At 10:00 hours every day, run the following command..."

And there you have it.  One randomly set desktop background in LXDE.

Incidentally, if you're curious about all the nifty things you can do with cron, I recommend playing around at, it's an online editor for crontab settings.  It's good for experimenting in such a way that you don't have to worry about messing up your system, and it's also handy for figuring out particularly arcane cronjobs.

Algorithm for implementing a dead man's switch.

Mar 04 2018

So, you're probably wondering why I'm posting this, because it's a bit off of my usual fare.  The reason is I think it would be useful to make available a fairly simple algorithm for implementing a general purpose dead man's switch in whatever language you want, which is to say a DMS that could conceivably do just about anything if it activated.

But what's a dead man's switch?  Ultimately, it's a mechanism that has to be manually engaged at all times if you want something to happen, and if that switch turns off for some reason, something else happens (like a failsafe).  A good example of this is the bar on the handle of a power lawnmower you have to hold down so it'll move while the engine's running.  If you let go of the bar the engine keeps running but the lawnmower doesn't keep rolling forward.  Another example can be found in locomotives; the conductor has to hold down a switch or lever so the engine will pull the train, and if that lever is ever let go (say the engineer has a heart attack or is otherwise incapacitated) the throttle closes and the train will grind to a halt.  More along the lines of what I'll be talking about are the watchdogs found in industrial controllers and realtime operating systems.  While running normally a software process inside the device flips a bit somehow - say, writing a 0 into a certain device node.  If the underlying hardware ever finds that the bit didn't get flipped within a certain period of time it reacts somehow to fix things (for example, it might reboot in an attempt to un-stick the gizmo).