computers (15)


How to disable spotlight on MacOS

So, few weeks after having this MacBook Air with horrid screen, I figured out that there is a small issue with RAM. 2 tasks permanently eating 2GB of RAM, effectively reducing 8GB of RAM this machine has, to only 6GB (which sadly isn’t enough these days, especially not for a power user, who wants to use Firefox and also some other programs – Mozilla expects that firefox is all you need, so it deliberately uses all free RAM you have :)).

The problem was with these 2:

  • Kernel task – obviously a kernel, but why does it need 1GB? I need to figure that out later
  • mdm_store – process belonging to “spotlight” which is some searching / suggestion feature I will never use

So I found some information about spotlight and people who wanted to disable to protect privacy. While I completely don’t care about that, I do care about my RAM, so here is how you can kill it permanently. Open a terminal and put there:

Here we go, 1GB of RAM back for use!




Optimizing Ansible for high performance

So, I have made a custom Ansible setup for more than 4000 servers in 12 different countries across the planet, and that gave me some insight into how to make it perform better.

First of all, sadly Ansible doesn’t yet support “proxy / caching servers” as in servers that you could use to execute playbook through. You can configure SSH proxy server, but that won’t help with performance. Only way to execute playbook from another server is to install Ansible there as well, sync the playbooks somehow and execute from this host.

Anyway, now for the performance hacks.

Redis caching

Major boost in performance. Simply install redis server on same host as Ansible and put this to configuration of ansible:

This will put all facts of every server you connect to into redis cache and next time you execute anything on that server (within 1 day), ansible will not gather facts again, but it would take them from redis cache.

Pipelining

Minor boost. But slightly helps:

Multithreading

Major boost, but not very stable, often causes troubles. Putting more than 20 makes Ansible quite unstable.

Example config

This config works pretty well to me:

 




How to install mdadm to XenServer 7

Based on https://discussions.citrix.com/topic/378478-xenserver-7-raid1-mdadm-after-install-running-system/

This post is basically just a backup of that forum post in case it become dead link




Letsencrypt kung-fu

Let’s encrypt CLI client is by far the most shittiest software ever invented, there is probably no doubt about it, but sadly, it’s the only interface that is supported, and unless you want to pay money for SSL certificate you need to live with that.

First of all – yes, their client (without asking or telling you) WILL run sudo and WILL use root and most likely WILL install garbage on your server that you don’t want to have there. If you never used letsencrypt client before, run it on testing VM first, before it desecrates your favorite web server with random garbage you don’t want there.

The letsencrypt client is written for dumb people, and it is based on undocumented black magic that I will try to uncover here a bit. The client basically works with a component called “certbot” which is a software that run on your server and does something to prove that you really own the domains for which you want to generate your SSL certificate. Because letsencrypt staff doesn’t want to bother you with technicalities they created this crap of a software to deal with them for you, in their own way, like it or not. It uses so called ACME (Automatic Certificate Management Environment) protocol to verify that you are owner. This thing is not a rocket science, and in a nutshell all it does is publish some data used to prove your ownership through your webserver, usually located on webroot/.well_known, their counter-party server will try to locate these by accessing your.domain/.well_known and in order to make it possible to verify your domain without modifications to your webserver, all you need to do is to create a central webroot and then make a symlink from all domain webroots to this one (just ln -s /var/www/letsencryptshite/.well_known /var/www/your.uber.tld/.well_known).

Once you do that, always pass these 2 parameters to their “software”:

I also strongly recommend you to maintain a comma separated list of all domains for which you want to get your certificate and store it somewhere like /etc/letsencrypt/domains because you will need to provide this list very often.

Now a little cheat sheet:

Renewing all domains

This can even be in your cron

You may need to restart / reload your web server after doing this, since the certificate will be overwritten, and Apache seems to be caching it somehow.

Adding or remove a domain and regenerate certificate

Modify your /etc/letsencrypt/domains list and run

Common locations:

/etc/letsencrypt – root of this thing’s config

/etc/letsencrypt/live – symlinks to current certificates, that’s where you can find chains for your domains

Example apache config that uses letsencrypt cert

 




How to create login info in motd similar to ubuntu server

Ever wondered how could you get this cool login screen you can see when you login to ubuntu server on other distros?

This is a part of proprietary system called landscape. But this thing is too cool to remain proprietary, so I created a similar login info screen here: http://github.com/benapetr/system-info

All you need to do is to append it to your login scripts and here we go:

 




Is internet ready for IPv6?

No, it’s not. Why?

I did an experiment as I was curious how various internet sites and service providers are ready for IPv6 by setting up a computer which supports IPv6 only. That means all sites which were not available through IPv6 just didn’t work.

It was pretty much every site that didn’t work, with few exceptions. Google, facebook and most of linux webpages (like debian.org) work just fine. Except for these, even most major websites such as twitter don’t support IPv6 yet, being completely inaccessible without IPv4 protocol.

Here is a small list of few examples

Major websites that support IPv6:

  • wikipedia.org
  • google.com
  • facebook.com

Major websites that can’t be accessed without IPv4:

  • github.com
  • twitter.com
  • bbc.co.uk
  • microsoft.com
  • ubuntu.com

 What is IPv6 and why should servers support it? From wikipedia:

Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4 address exhaustion. IPv6 is intended to replace IPv4.

In a nutshell: it’s a new version of internet protocol that will replace IPv4 someday, the main reason for it is the low number of IPv4 addresses, because its protocol was invented in 1980 by engineers who never expected internet to grow up into this size. IPv4 supports less than 4,294,967,296 network addresses, which may look as enough to some, but is actually very small number, given a number of network enabled devices out there (every modern TV, mobile phone or PC).

It’s totally possible to connect many more devices to IPv4 based internet, but only as long as they are grouped behind NAT servers with shared IP addresses. This will work for regular users who don’t need much from internet, but definitely will not for advanced users who want to use internet in order to provide some services to others, or those who want their devices to be easily reachable from anywhere through internet.

Right now, public IPv4 address is something that actually has some value. You have to pay for it, if you want to have it. IPv6 addresses are so cheap that most of ISP providers would give you whole range of public IPv6 addresses for free.

That means that sooner or later various people will start up services (web servers, game servers and so on) that would be available only over IPv6 because they would need to use public address(es), but they wouldn’t want to pay for them, because they don’t have to.

These IPv6 only servers would not just be available only to people with IPv6, but they would also be only able to connect to machines (and other servers) that support only IPv6. And that is the reason why your system should be IPv6 ready.

How do I check if my website works on IPv6?

It’s easy, just run this ipv6 test

If it doesn’t, don’t worry. Anything like “switchover” to IPv6 isn’t going to happen any time soon, so there is still plenty of time. Regular users will (have to) keep using IPv4 for a very long time…




How to setup debian build environment for both amd64 and x86 platform

This article describes how to setup environment in which you can build both amd64 as well x86 .deb packages. It’s assuming that you already have working debian on amd64 kernel.

First step: install debootstrap

Now you should have fully working x86 debian in /opt/jessie-386

Second step: make a script to switch into i386 version

Create a script anywhere you like, for example /bin/switch_32 with following content:

Then make it executable. Now running it would switch you into fully working debian x86 in which you can install all required packages using apt-get and build your x86 packages.




PowerShell syntax cheat sheet

Everything is case insensitive, comments with # symbol

Conditions

if (condition)
{
   #code
} else if (condition)
{
   #code
} else
{
   #code
}

if (condition1 -And condition2 -And !(condition3))
{}

Negations must be in extra brackets ex:

$x = $true
if ($x) {}
if (!($x)) {}

Parameters

There is special block of code in param, that must be first executable code in function or script. Can be after comments.

Example:

#This is a beginning of script file

param
(
        [string] $PackageName = “blah”,

        [parameter(Mandatory=$true)]     
        [ValidateNotNullOrEmpty()]
        [string] $PackageUrl,

        [parameter(Mandatory=$true)]
        [string] $VariableName
)

You can call the script or function, either with parameters in order in which they are defined like

blah a b c

or with switch

blah -PackageName “a” -PackageUrl “b” -VariableName “c”

Types

[string] $x = “blah”
[bool] $x = $false #or $true
#store output of a command into string
$rev_list = git rev-list HEAD –count | Out-String 
#replace new line
$rev_list = $rev_list.Replace(“n", "").Replace("r”, “”) 

Functions

# Example function with example call of that function

$ErrorActionPreference = “Stop”

function PackageTest
{
    param
    (
        [parameter(Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string] $PackageName,

        [parameter(Mandatory=$true)]     
        [ValidateNotNullOrEmpty()]
        [string] $PackageUrl,

        [parameter(Mandatory=$true)]
        [string] $VariableName
    )

    Write-Host “Looking for $PackageName…    ” -NoNewLine
    if (!(Test-Path $PackageUrl))
    {
        echo “ERROR”
        echo “Unable to find $PackageName at $PackageUrl, you can set the alternative path using -$VariableName=path”
        exit 1
    }
    echo (“OK”);
}

PackageTest “Qt5” “$qt5_path” “qt5_path”




How to install mdadm on citrix xen 6.5

For some reason citrix doesn’t like mdadm so they make everything possible to stop it from working on their xen server.

Here is a guide that would make it work there, but it may not survive system patching

Setup

Connect at least 2 disks to your box. Install a xen server without local storage on first disk.

Installing mdadm

The default install contains mdadm but it doesn’t load raid modules to kernel. In order to enable it, following needs to be done:

Partitioning the disks

Now we create a final schema we want to use on our server on disk /dev/sdb, xen needs to have at least 3 partitions, 1 is for boot loader, second is for OS, I recommend 20gb or more, because this disk is pretty much impossible to extend, although citrix defaults it to 4GB, last partition is for local storage and it should take all remaining space on disk.

Note: Citrix by default creates 3 partitions, 1 for OS, second is empty, same sized as first one and probably used for system upgrade. Third is used for local storage LVM. You don’t have to create second partition for it to work, but system upgrades may not be available if you don’t create it. On other hand system upgrades will likely not work anyway as citrix doesn’t support mdadm installations.

In this guide I will use old MS-DOS partition table because although it’s old, it’s much better supported and it just works. You can also use GPT partitions if you want, but I had some issues getting them work with mdadm and syslinux.

We will have a separate /boot partition for boot loader, because syslinux shipped with xen is having troubles booting from raid device for some reason.

So this is how the layout of sdb should look after we finish the partitioning:

  • /dev/sdb1 (2 GB) for bootloader
  • /dev/sdb2 (20 GB) for OS
  • /dev/sdb3 (rest) for LVM

Now you should be able to boot from /dev/sdb if you are not there is something wrong with the setup, you need to figure out if your problem is with

  • MBR (No bootable device)
  • Boot loader (Missing operating system.)
  • /boot (Linux will start booting but die in progress – try removing quiet and splash from parameters)

 Syncing the disks

Now if you were able to boot up you need to setup the sda disk

Create the same 3 partitions as you did on sdb on sda and then

 




Why writing verbose code is good for self documentation

I recently had an interesting discussion about modern programming languages and I also have seen some, rather weird, opinions of people who believe that being verbose in your source code is a bad thing. I will try to summarize in this post why actually being verbose is a good and useful thing.

First of all, what do I mean by being verbose? I mean to use language elements that can be, but not necessarily need to be explicitly stated in the source code.

Many languages have some implicit definitions that are always used, unless you override them. For example in c# these both lines are identical:

Because every function, if not stated otherwise is private be default. There is no need to use keyword “private”. But well, you can still do that. Why? Because not everybody knows it and that doesn’t necessarily mean that person is a newbie (there are people who simply work in so many languages that they are occasionally unclear about things like this), and even if they were, what is so wrong on writing a source code that is simple to read even by newbies? So in simple words this will make it extra clear that a function is really private. It doesn’t harm anything and may be useful to some people who are reading the code.

The other thing which I noticed, is that people usually do not like to use “this” in c++. I am personally using “this” everywhere I can, in every function, just to make it 100% clear that this variable or a function is a non-static member of instance of class in which the function body is defined. It’s not for me, it’s for people who are reading my code, so that they don’t need to scroll potentially hundreds lines of code just to figure out whether variable, which they are looking at is function local or belongs to the class. I know that some IDE’s are showing these in different color, so that their scope is pretty clear, but these people may be reading this code in VI, or notepad. Again, it doesn’t harm anything, there is no difference in binary produced by compiler. In addition, it helps to ensure that your call will not conflict with some other function with same name.

It doesn’t improve the code, it doesn’t make it better nor worse. It just makes it easier to read (not by you, but by someone who doesn’t know the code and wants to understand it).

And what I hate most are languages which doesn’t require explicit type defintion, but are still strong-typed, like python (yes you probably already know I hate python :-)). These languages makes it super hard to figure out which datatype you are looking at, unless you attempt to debug it in some way.

This may be good for obfuscation, but seriously, what’s the real benefit of programming language that allows you to write shortest possible source code in number of characters which is totally unclear and hard to understand? To save space on hard drive? I don’t think so…