Your problem is probably not the amount of data you store, but the number of
SSTable files.  When these increase, read latency goes up.  Write latency maybe
goes up because of compaction.  Check in the data directory, whether there are many
data files, and check via JMX whether compaction is happening.
My recommendation is to reduce the write traffic to the nodes so that each node can
keep up with compaction.  If reducing the load is not possible, you have to add nodes
(or get faster hard disks, but that is often not possible).

From: hive13 Wong []
Sent: Thursday, June 10, 2010 2:58 PM
Subject: single node capacity


How much data load can a single typical cassandra instance handle?
It seems like we are getting into trouble when one of our node's load grows to bigger than 200g. Both read latency and write latency are increasing, varying from 10 to several thousand milliseconds.
machine config is 16*cpu 32G RAM 
Heap size is 10G
Any suggestion of tuning?
Or should I start considering adding more nodes when the data grows to this big?