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From Kamil Gorlo <>
Subject Cassandra performance
Date Tue, 14 Sep 2010 22:40:05 GMT

we are considering using Cassandra for quite large project and because
of that I made some tests with Cassandra. I was testing performance
and stability mainly.

My main tool was for benchmarks (or equivalent written in
C++ to deal with python2.5 lack of multiprocessing). I will focus only
on reads (random with normal distribution, what is default in because writes were /quite/ good.

I have 8 machines (xen quests with dedicated pair of 2TB SATA disks
combined in RAID-O for every guest). Every machine has 4 individual
cores of 2.4 Ghz and 4GB RAM.

Cassandra commitlog and data dirs were on the same disk, I gave 2.5GB
for Heap for Cassandra, key and row cached were disabled (standard
Keyspace1 schema, all tests use Standard1 CF). All other options were
defaults. I've disabled cache because I was testing random (or semi
random - normal distribution) reads so it wouldnt help so much (and
also because 4GB of RAM is not a lot).

For first test I installed Cassandra on only one machine to test it
and remember results for further comparisons with large cluster and
other DBs.

1) RF was set to 1. I've inserted ~20GB of data (this is number
reported in load column form nodetool ring output) using
(100 colums per row). Then I've tested reads and got 200 rows/second
(reading 100 columns per row, CL=ONE, disks were bottleneck, util was
100%). There was no other operation pending during reads (compaction,
insertion, etc..).

2) So I moved to bigger cluster, with 8 machines and RF set to 2. I've
inserted about ~20GB data per node (so 20 GB * 8 / 2 = 80GB of "real
data"). Then I've tested reads, exactly te same way as before, and got
about 450 rows/second (reading 100 columns (but reading only 1 in fact
makes no difference), CL=ONE, disks on every machine was 100% util
because of random reads).

3) Then I changed RF from 2 to 3 on cluster described in 2). So I
ended with every node loaded with about 30GB of data. Then as usual,
I've tested reads, and got only 300 rows/second from whole cluster
(100% util on every disk).

4) Last test was with RF=3 as before, but I've inserted even more
data, so every node on 8-machines cluster had ~100GB of data (8 *
100GB / 3 = 266GB of real data). In this case I've got only 125

I was using multiple processes and machines to test reads.

*So my question is why these numbers are so low? What is especially
suprising for me is that changing RF from 2 to 3 drops performance
from 450 to 300 reads per second. Is this because of read repair?*

PS. To compare Cassandra performance with other DBs, I've also tested
MySQL with almost exact data (one table with two columns, key (int PK)
and value(VARCHAR(500))  simulating 100 columns in Cassandra for
single row). MySQL was installed on the same machine as Cassandra from
test 1) (which is one of these 8 machines described before). I've
inserted some data and then tested random reads (which was even worse
for caching because I've used standard rand() from C++ to generate
keys, not normal distribution). Here are results:

size of data in db -> reads per second
21 GB  -> 340
400 GB -> 200

So I've got more reads from single MySQL with 400GB of data than from
8 machines storing about 266GB. This doesn't look good. What am I
doing wrong? :)


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