Speaker: Andy Cobley, Lecturer at University of Dundee Video: http://www.youtube.com/watch?v=0U4iOSMnRdk&list=PLqcm6qE9lgKLoYaakl3YwIWP4hmGsHm5e&index=1 Abstract: The raspberry Pi is a credit-card sized $25 ARM based linux box designed to teach children the basics of programming. The machine comes with a 700MHz ARM and 512Mb of memory and boots off a SD card, not much power for running the likes of a Cassandra cluster. This presentation will discuss the problems of getting Cassandra up and running on the Pi and will answer the all important question: Why on Earth would you want to do this!?

1. Hardware Agnostic: Cassandra on Raspberry Pi
Andy Cobley | Lecturer, University of Dundee, Scotland
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2. What we will discuss today…
* 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?
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3. Who Am I ?
* Andy Cobley
* Program Director, MSc in Data Science and Business Intelligence
* School of Computing
* University of Dundee
* Twitter: @andycobley
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4. Whats a Raspberry Pi ?
* Single chip Linux computer
* 500 Meg ram
* Boots off an SD card
* Ethernet port
* (graphics and all you need for a general purpose computer)
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5. Pi with pound coin
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6. And, here’s one for real
* Also 4 node cluster.
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7. The Bad News
* 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 !
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8. More bad news !
* Running a external USB drive is actually worse !
* Probably be hardware feature
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9. Raspberry Pi Schematic
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10. And then there’s Java!
* Oracle Java vs OpenJDK
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11. And Raspbian
* Raspbian is Debian for the PI
* Uses the Hard floating point accelerator
* Much faster than Debian
* Current official Oracle JDK won’t run on it !
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12. Oracle java
* http://www.oracle.com/technetwork/java/embedded/downloads/
javase/index.html
* Java SE Embedded version 6
* Cassandra might prefer 6 (or 7 for Cassandra 2)
* But
* https://blogs.oracle.com/henrik/entry/oracle_releases_jdk_for_linux
* Preview at:
* https://jdk8.java.net/fxarmpreview/
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13. Hard vs Soft Float
* And then it turns out:
Actually
not
much
difference
in
performance
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14. The Problem with compression
* Cassandra uses compression for performance
* Started in version 1.0
2x-­‐4x
reduc8on
in
data
size
25-­‐35%
performance
improvement
on
reads
5-­‐10%
performance
improvement
on
writes
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15. Compression types
* Three types:
Google
Snappy
Compressor
(Faster
read/writes)
DeflateCompressor
(Java
zip,
slower
,
beOer
compression)
* Snappy Compression not available on Pi
(requires
na8ve
methods,
so
someone
might
get
it
to
work!)
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16. Compression
* Cassandra 1.2 (and 2) also has lz4 compression
* Which is good news !
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17. And the startup script
* Startup script allocates memory
* Calculates based on number of processors
* Pi reports Zero processors !
* Boom !
* Now fixed
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18. JMX Config
* In Cassandra-env.sh
* JVM_OPTS="$JVM_OPTS Djava.rmi.server.hostname=192.168.1.15”
* Or else nodetool will not work between nodes
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19. JVM OPT UseCondCardMark
* C* 1.22. added UseCondCardMark as a JVM Opt
* "for better lock handling especially on hotspot with multicore
processor”
* In cassandra-env.sh
#if
[
"$JVM_VERSION"
>
"1.7"
]
;
then
#
JVM_OPTS="$JVM_OPTS
-­‐XX:
+UseCondCardMark"
#fi
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20. #CASSANDRAEU
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21. The Good News !
* 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:
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22. So, have a 64 node computer for £2000
University
of
Southhampton
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23. Or this
* 32 node Beowolf cluster:
* Joshua Kiepert, Boise University
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24. Or this Hadoop Cluster from LinkedIn
hOp://prac8calcloudcompu8ng.com/post/53996976003/hadoop-­‐running-­‐on-­‐a-­‐14-­‐chip-­‐
raspberry-­‐pi-­‐cluster
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25. Adding nodes is good
* 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.
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26. Vnodes
* Vnodes (in 1.2) would be very nice
* However at this point I haven’t got 1.2 on Pi running on a cluster
* As for Cassandra 2, see later
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27. Performance with 3/4 nodes
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28. Performance with 5/6 nodes
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29. Stress test commands
* ./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
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30. Getting more memory
* 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
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31. Getting more Memory
* Under Rasbian
* Run with a monitor plugged for the first time
* Set options for screen memory
* Perhaps disable boot to GUI
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32. Network address
* I prefer static network addresses
* Edit /etc/network/interfaces
iface
eth0
inet
sta8c
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
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33. Multiple nodes
* 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)
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34. Starting as a service
* See https://github.com/acobley/CassandraStartup
* Put the file in /etc/init.d
* update-rc.d cassandra defaults
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35. Pi is for teaching
* 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 students will use these clusters
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36. C* is network aware
* 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
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37. Proposed teaching tool
Noise
injec8on
10mbs
Hubb
Switch
1
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Switch
2
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38. TC ?
* What about the Linux tc command
* Lets look again at the diagram
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39. Network
* What we can’t do
* Recommended bandwidth is 1000 Mbit/s (Gigabit) or greater.
* Bind the Thrift interface (listen_address) to a specific NIC (Network
Interface Card).
* Bind the RPC server interface (rpc_address) to another NIC.
hOp://www.datastax.com/docs/1.2/cluster_architecture/cluster_planning
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40. What about Cassandra 2.0
* Internode compression currently uses Snappy
* So turn it off in conf file:
internode_compression:
none
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41. How does C* 2 run on a PI
* Some bad news
* So need to tune it :
* See John Berryman’s blog:
* http://www.opensourceconnections.com/2013/08/31/building-theperfect-cassandra-test-environment/
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42. Pi is discovery
* 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
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43. Pi is for fun
* Most important, this was pure Geeky Fun
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44. Obligatory Plug
* Data Science:
* http://www.computing.dundee.ac.uk/study/postgrad/
degreedetails.asp?17
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45. What we discussed today…
* Raspberry Pi is cheap
* C* needs some work to run on it
* You can make clusters cheaply for experimentation
* It’s fun !
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46. THANK YOU
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