short lighting talk on Apache Cassandra Anti-Patterns from Cassandra SF 2011

1. Cassandra Anti-Patterns (in 5m)
Matthew F. Dennis // @mdennis

2. Non-Sun (err, Non-Oracle) JVM
● No OpenJDK
● No Blackdown (anyone still use this?)
● Etc, etc, etc; just use the Sun (Oracle) JVM
● At least u22, but in general the latest release
(unless you have specific reasons otherwise)

3. CommitLog+Data On The Same Disk
● Don't put the commit log and data directories on
the same set of spindles
– commit log gets a single spindle entirely to itself (standard
consumer SATA disks easily sustain > 80 MB/s in
sequential writes)
● DOES NOT APPLY TO SSDS or EC2
● SSDs have no seek time
● EC2 ephemeral drives are still virtualized (but not the
same as EBS)
● On EC2 or SSDs: use one RAID set for both the
commit log and data directories

4. EBS volumes on EC2
● Sounds great, nice feature set, but …
● Not predictable
● “freezes” are common
● Throughput limited in many cases
● Use ephemeral drives instead
● Stripe them
● Both commit log and data directory on the same
raid set

5. Oversized JVM heaps
● 6 – 8 GB is good (assuming sufficient ram on
your boxen)
● 10 – 12 GB is possible and in some
circumstances “correct”
● 16GB == max JVM heap size
● > 16GB => badness
● JVM heap ~= boxen RAM => badness (always)

6. JVM heap size -v- GC suckage
~16GB
GC Suckage
~10GB
~6GB
JVM heap size

7. Large batch mutations
(large in number of distinct rows)
● Timeout / failure => entire mutation must be
retried => wasted work
● Larger mutations => higher likely hood of
timehood
● 1000 mutations to perform? Do 100 batches of
10 in parallel instead of one batch of 1000
● Exact number or rows/batch is variable
depending on HW, network, load, etc;
experiment! (10-100 is a good starting point)

8. OPP / BOP partitioner
● You probably shouldn't use it
● No really, you almost certainly shouldn't use it
● Creates hot spots
● Requires “baby sitting” from ops
● Not as well tested nor is it widely deployed

9. C* auto selection of tokens
● Always specify your initial token.
● Auto select doesn't do what you think it does
nor does it do what you want
– loadbalance is even worse, it doesn't currently do what
you think, what you want or what it claims; “F#@* my
cluster” would be a much more apt name than
“loadbalance”
– Future (next?) release of OPSC will remove your
balancing woes

10. Super Columns
● 10 – 15 percent performance penalty on reads and writes
● Easier / better to use to composite columns
– 0.8.x makes this a lot easier
– Done manually in 0.7.x and is still better
● Devs working in C* code despise (loathe?) them
● API probably won't be deprecated, but implementation will be
replaced behind the seen with composites (may be “ok” at that point
to use them, but should probably just use composite API direclty)
● Cassandra and DataStax is committed to maintain the API going
forward, even if the implementation changes

11. Read Before Write
● Race conditions
● Abuses/Thrashes cache (row, key and page)
● Increases latency
● Increases IO requirements (by a lot)
● Increases size in the client

12. Winblows
● Try to avoid it, you'll be happier
– Not always possible? Then, “I'm sorry for your pain”
● Run 'nix (in particular, probably Linux)
● Easier to get help (IRC, email, meetups, etc)
● C* performs better
● Better tested
● Cheaper
● Wider deployed (by a lot)

13. Q?
Cassandra Anti-Patterns
Matthew F. Dennis // @mdennis