07 May 2015

On Debian's init process

Are you tired of badly written, complex, buggy and generally shitty init systems on Debian based systems?
Do you wonder what certain Debian developers have been smoking lately?
Do you think upstart, systemd and the likes need to stay the fuck away from your otherwise pretty stable Debian systems and do you long for the simpler SysV days?
Yeh, me too.

apt-get install sysvinit systemd-
or

apt-get install sysvinit upstart-

takes care of it.
That's all.

29 April 2015

On engine braking

I'm not sure what engine braking is.

I know what it does and I can apply the effect, but I'm not sure what causes it, the physics behind it, you know.
And I want to know because I want to understand why 2T has got so little of it and why it's configurable on 4T engines.

Yes, engine braking is a configurable setting on some/most/all (?) ECU equipped bikes.
I have never seen any manufacturer offer ECU programming software on off-the-shelf bikes, so I guess most people have never seen it's configurable; but at least YEC and HRC offer it on their race-spec ECU's and I can't imagine the others wouldn't.

Anyways, let's see what the internet can tell me about engine braking.

From Wikipedia:
"The term 'engine braking' refers to the braking effect caused by the closed-throttle partial-vacuum in petrol (gasoline) engines when the accelerator pedal is released. While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from the vacuum.

When the throttle is closed, the air flow to the intake manifold is greatly restricted. The concept can be illustrated by the amount of effort required to blow/suck through a thin tube vs. a wider one. It is the work the engine has to do against this restricted air flow that provides the braking effect."

"As soon as the accelerator is released and the throttle closes, engine braking comes into effect as long as the wheels remain connected via the transmission to the engine."

"The braking force varies depending on the engine, but also what gear the vehicle is in."

"Additionally, fuel injected engines don't use any fuel while engine braking. This is known as DFCO or Deceleration Fuel Cut-Off."


Doesn't explicitly state what causes it, but at least I've extracted some key points:
  • no fuel usage during engine braking
  • lack of air flow makes the 'engine' do something to invoke engine braking
  • engine braking only occurs when the drive train is connected to the engine and the tire connected to the road
  • transmission plays a part (the lower the gear -> the higher the engine braking)

I'm going to assume it's the exact opposite of accelerating (I've got to start somewhere figuring this out), which makes the slowing down effect must come from the 'engine' as the Wikipedia so eloquently states. But what is the 'engine' in this case?

Acceleration comes from the down movement of the piston in the cylinder due to the rapidly expanding gas mixture combustion inside a vacuum chamber.
  • The vacuum chamber comes from all the valves being closed as being one phase in a four-stroke's cycle.
  • The mixture comes from air/fuel being mixed and injected.
  • The combustion comes from from the spark plug igniting the gas/air mixture
But we're not accelerating, we're engine braking.

Maybe that's it: the fact that during that exact stroke, everything is absent that otherwise makes it accelerate:
  • we've closed the throttle, there is no gas/air mixture in the combustion chamber.
  • there is no spark to ignite anything
  • there is no compression, there is just vacuum
That's it! Expanding a vacuum is really hard, right?
The drive train still is connected throughout the entire time.
Inertia makes the back wheel keeps moving so everything connected to the wheel is moving, still.
The chain is moving, the transmission is moving and ultimately, the piston itself it going up and down still.
And that must be a pretty hard thing to do, the wheel pulling the piston down trying to expand the vacuum inside the combustion chamber and there's the deceleration effect right there.

Now I get it!
It's configurable because the injection system can still inject a certain amount of air inside the chamber right before the 'pull' to make it easier to expand the other wise semi-vacuum chamber.
The more air injected before the 'pull' stroke, the easier it'll be to pull down on the whole, the less pronounced the engine brake effect will be.

And now I get why 2-strokes haven't got as much engine braking, to the point where it's claimed there's none at all.
It's because in a 4T system, there's a completely separate phase where the entire system is closed down, creating a lot of time for the back wheel to try and pull the piston down, expanding the semi-vacuum and that never really happens in a 2T system. 
It kind of does, but it doesn't nearly last as long as in a 4T system, making the engine brake effect negligible.

20 May 2014

Bancontact mobile application with a Belgian Keytrade account

It took some time to get it working; if the 'activation' step in the mobile application keeps failing, do this:
Log into the Keytrade web site and go to 'Cards' -> 'Card Security' -> 'Activate Card for Online Payments'.
That's it.

Screenshot because it's fancy:


19 March 2014

Unsetting a trap

Quick one: catching a trap in bash is fairly easy with bash' trap command (search for 'trap \[' in bash' man page) but how to clear a set one?
It's in the man page, of course, but who reads those, right? I'm betting the answer is bound to be found on some blog through Google in 0.76 seconds flat ;)
"If arg is absent (and there is a single sigspec) or -, each specified signal is reset to its original disposition (the value it had upon entrance to the shell)."
So it turns out it's fairly simple and this demonstrates it:
root@mgmtsrv:~# cat test.sh
#!/bin/bash
trap 'echo TRAPPED!; break' SIGINT
while [ True ]; do
        sleep 1
done
echo 'end of script'
exit 0
root@mgmtsrv:~# ./test.sh
^CTRAPPED!
end of script
root@mgmtsrv:~#
Now, let's unset the trap:

root@mgmtsrv:~# cat test.sh
#!/bin/bash
trap 'echo TRAPPED!; break' SIGINT
while [ True ]; do
        sleep 1
done
trap - SIGINT
while [ True ]; do
        sleep 1
done
echo 'end of script'
exit 0
root@mgmtsrv:~# ./test.sh
^CTRAPPED!
^C
root@mgmtsrv:~#
So, in short: the 'trap - SIGINT' line clears the set trap :)

05 February 2014

On loopback device creation

I was having some problems with loop back device creating & almost immediately detaching; it seems sometimes a kernel thread holds on to the device and the detach fails.
I decided to have a look what's going on so I created a script which automates the creation, attaching and detaching of loopback devices:


# ./test.sh

Creating sparse 4TB file: Done
----START----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
Checking for free loopback device: /dev/loop1
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
Checking for free loopback device: /dev/loop2
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
/dev/loop2
Checking for free loopback device: /dev/loop3
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
Checking for free loopback device: /dev/loop4
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
/dev/loop4
Checking for free loopback device: /dev/loop5
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
/dev/loop4
/dev/loop5
Checking for free loopback device: /dev/loop6
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
/dev/loop4
/dev/loop5
/dev/loop6
Checking for free loopback device: /dev/loop7
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
----NEXT PASS----
Loopback devices currently in use:
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
/dev/loop4
/dev/loop5
/dev/loop6
/dev/loop7
Checking for free loopback device: losetup: could not find any free loop device
Exiting, couldn't find any free device any more.
So if done fast enough the pool quickly gets depleted as giving back (detaching) the device fails. A second or so later it might work, though. But let's test how much time exactly is needed to free the device? I modified the script to keep trying to detach/free the loopback device and see what happens:

Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 4 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 6 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 3 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 3 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.

Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 3 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 4 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 4 tries.
----NEXT PASS----
Loopback devices currently in use:

Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 5 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 10 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 4 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 4 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 3 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
Loopback device broken down after 0 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 4 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 3 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
Loopback device broken down after 0 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 3 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 2 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: ^C
So it varies. The device might get released instantaneously, but that doesn't happen too often, or it might take 10 consecutive tries. Let's see if allowing half a second's worth of pause in between creating and detaching fix the problem,

# ./test.sh
Creating sparse 4TB file: Done
----START----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0.5 seconds.
Breaking down loopback device:
Loopback device broken down after 0 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0.5 seconds.
Breaking down loopback device:
Loopback device broken down after 0 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0.5 seconds.
Breaking down loopback device:
Loopback device broken down after 0 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0.5 seconds.
Breaking down loopback device:
Loopback device broken down after 0 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0.5 seconds.

^C
It does, but what's holding on to the device exactly? I modified the script again to look up the did in the process table. This takes time of course, so the detaching phase took less tries to complete:

# ./test.sh
Creating sparse 4TB file: Done
----START----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
root     25417  0.0  0.0      0     0 ?        S<   13:25   0:00 [loop0]
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
Checking for free loopback device: /dev/loop0
Setting up loopback device: Done
Sleeping for 0 seconds.
Breaking down loopback device:
loop: can't delete device /dev/loop0: Device or resource busy
root     25437  0.0  0.0      0     0 ?        S<   13:25   0:00 [loop0]
Loopback device broken down after 1 tries.
----NEXT PASS----
Loopback devices currently in use:
^C
Right, so it's Linux that's holding on to it and I'm not sure if I like this behavior. I realize losetup can only request Linux to free up the device and it does cleanly report a 'Device or resource busy' but I don't understand why it just doesn't wait a bit until the resource is cleared from the kernel, or at least provides the option to the user to do so. I thought about pathing losetup but now I can't be bothered anymore, so I'm just putting it out there :)

18 December 2013

On bypassing mdadm's interactive mode

So I wanted mdadm to not go interactive, ever, but instead just bail out whenever something happens it doesn't understand; you'd think the developers would have thought of that, right? Turns out they haven't.
The 'fix' is easy enough though: pipe /bin/false through mdadm by default so it'll get a 'false' every time it would be looking for user input; effectively making it bail out whenever something's off:

alias mdadm="/bin/false | /sbin/mdadm"

04 December 2013

On quickly creating sparse and large files

I've been using dd(1) for as long as I can remember to create sparse files but for some reason I keep forgetting how to use dd's argument options so I set out to find something better... and found it: truncate(1):

From the man page: "Shrink or extend the size of each FILE to the specified size. A FILE argument that does not exist is created." -looking good:

root@debian64:~# time truncate -s 10T testfile

real0m0.003s
user0m0.000s
sys 0m0.004s

root@debian64:~# ls -l testfile
-rw-r--r-- 1 root root 10995116277760 Dec4 10:27 testfile

root@debian64:~# du -hs testfile
0 testfile

Awesome!
So then I set out to find me something to quickly allocate 'real' files:

fallocate(1):
"fallocate  is used to preallocate blocks to a file.  For filesystems which support the fallocate system call, this is done quickly by allocating blocks and marking them as uninitialized, requiring no IO to the data blocks.  This is much faster than creating a file by filling it with zeros."

And it truly is fast:


root@debian64:~# time fallocate -l 1G testfile

real    0m0.004s
user    0m0.000s
sys     0m0.000s

root@debian64:~# du -hs testfile
1.1G    testfile

root@debian64:~# ls -l testfile
-rw-r--r-- 1 root root 1073741824 Dec  4 10:34 testfile