1. Hardware Agnostic: Cassandra on Raspberry Pi
Andy Cobley | Lecturer, University of Dundee, Scotland

2. *Cassandra is hardware agnostic
*So why not run it on a Raspberry Pi ?
*How hard can it be ?
*What can we do with it once it works?
Cassandra on Raspberry Pi

3. *Andy Cobley
*School of Computing
*University of Dundee
*Twitter: @andycobley
Who Am I ?

4. *Single chip Linux computer
*500 Meg ram
*Boots off an SD card
*Ethernet port
*(graphics and all you need for a general purpose computer)
Whats a Raspberry Pi ?

5. Pi with pound coin

6. *And here’s the Cassandra cluster *
And, here’s one for real
* Power Permitting !

7. *Cassandra is designed to be fast, fast at writing, fast at reading.
*This laptop with one instance of Cassandra will do 12,000 write
operations
*Raspberry Pi will do 200 !
The Bad News

8. *Running a external USB drive is actually worse !
*Probably be hardware feature
More bad news !

9. Raspberry Pi Schematic

10. *Oracle Java vs OpenJDK
And then there’s Java!

11. *Raspbian is Debian for the PI
*Uses the Hard floating point accelerator
*Much faster than Debian
*Current Oracle JDK won’t run on it !
And Raspbian

12. *http://www.oracle.com/technetwork/java/embedded/downloads/java
se/index.html
*Java SE Embedded version 6
*Cassandra might prefer 6
*But
*https://blogs.oracle.com/henrik/entry/oracle_releases_jdk_for_linux
*Preview at:
*https://jdk8.java.net/fxarmpreview/
Oracle java

13. *Actually not much difference in performance
Hard vs Soft Float

14. *Cassandra uses compression for performance
*Started in version 1.0
2x-4x reduction in data size
25-35% performance improvement on reads
5-10% performance improvement on writes
The Problem with compression

15. *Two types:
Google Snappy Compressor (Faster read/writes)
DeflateCompressor (Java zip, slower , better
compression)
*Snappy Compression not available on Pi
(requires native methods, so someone might get it to
work!)
Compression types

16. *Startup script allocates memory
*Calculates based on number of processors
*Pi reports Zero processors !
*Boom !
*Now fixed
And the startup script

17. *In Cassandra-env.sh
*JVM_OPTS="$JVM_OPTS -
Djava.rmi.server.hostname=192.168.1.15”
*Or else nodetool will not work between nodes
JMX Config

18. *C* 1.22. added UseCondCardMark as a JVM Opt
*"for better lock handling especially on hotspot with multicore
processor”
*In cassandra-env.sh
#if [ "$JVM_VERSION" > "1.7" ] ; then
# JVM_OPTS="$JVM_OPTS -
XX:+UseCondCardMark"
#fi
JVM OPT UseCondCardMark

20. *We’ve forgotten one thing
*The Pi cost £25
*You can power 4 from USB hub (no need for a power supply on
each one)
*So:
The Good News !

21. So, have a 64 node computer for £2000
University of Southhampton

22. *32 node Beowolf cluster:
*Joshua Kiepert, Boise University
Or this

23. *Adding nodes adds performance
*Adding nodes adds replicas of data
*BUT
*Make sure your ring is balanced,
*Pi’s don’t like to be unbalanced.
Adding nodes is good

24. *Vnodes (in 1.2) would be very nice
*However at this point I haven’t got 1.2 on Pi running on a cluster
Vnodes

25. Performance with 3/4 nodes

26. Performance with 5/6 nodes

27. *./stress -d 192.168.1.10,192.168.1.11,192.168.1.12 -o insert -I
DeflateCompressor
*Note: nodes to use
*You will get different performance if you insert to less nodes than
you have in your ring
Stress test commands

28. *Adding a node (in the absence of Vnodes)
Must seed form a known node
Use a program to calculate new keys
Bring up new node with the correct key in
cassandra.yaml
Use node tool to move other nodes
Adding Nodes Procedure

29. *Python code
import sys
if (len(sys.argv) > 1):
num = int(sys.argv[1])
else:
num = int(raw_input("How many nodes? :"))
for i in range(0,num):
print 'node %d: %d' % (i, (i*(2**127)/num))
Calculating keys

30. *Use nodetool
sudo ./nodetool -h 192.168.1.10 move
42535295865117307932921825928971026432
*And cleanup
./nodetool -h 192.168.1.10 cleanup
Moving existing nodes

31. *On Debian, you can free memory from the graphics chip
Cd /boot
sudo cp start.elf start.elf.old
sudo cp arm224_start.elf to start.elf
reboot
Getting more memory

32. *Under Rasbian
*Run with a monitor plugged for the first time
*Set options for screen memory
*Perhaps disable boot to GUI
Getting more Memory

33. *I prefer static network addresses
*Edit /etc/network/interfaces
iface eth0 inet static
address 192.168.1.41
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.254
*
Network address

34. *Make a master SD card
*Copy it !
*Make sure the master version has no data on it.
*Consider ”Puppet” (though I don’t use it)
Multiple nodes

35. *See https://github.com/acobley/CassandraStartup
*Put the file in /etc/init.d
*update-rc.d cassandra defaults
Starting as a service

36. *So for £200 we get an 8 node C* cluster
*It can be reconfigured, blown away, stress tested and generally
abused
*We can simulate data racks, data centers and I hope even long
network delays.
*Hopefully our upcoming MSc in Data Science will use these clusters
Pi is for teaching

37. *We know C* can be configured to be aware of:
Network racks
Data Centers
*We know we can have replicas are stored across these racks
*How can we play with this cheaply ?
C* is network aware

38. Proposed teaching tool
10mbs
Hubb
Noise
injection
Switch
2
Switch
1
Pi 1
Pi 2
Pi 3
Pi 1
Pi 2
Pi 3

39. *Cassandra wouldn’t run on a PI
*It does now.
*Running it on a Pi shook out some Cassandra bugs
*You can run it in a secure lab
Pi is discovery

40. *Most important, this was pure Geeky Fun
Pi is for fun

41. *Data Science:
*http://www.computing.dundee.ac.uk/study/postgrad/degreedetails.a
sp?17
Obligatory Plug

42. *Raspberry Pi is cheap
*C* needs some work to run on it
*You can make clusters cheaply for experimentation
*It’s fun !
C* is Hardware Agnostic

43. THANK YOU