How to configure cassandra on EC2

I read most of the tutorials how to install cassandra in ec2 but I am very confused about the paths they recommend to store the data. In my ec2 instances my partitions are like below. Note: my rootfs is very small only 6GB.

Filesystem      Size  Used Avail Use% Mounted on
rootfs          6.0G  5.5G  513M  92% /
proc               0     0     0    - /proc
sysfs              0     0     0    - /sys
devtmpfs         15G     0   15G   0% /dev
securityfs         0     0     0    - /sys/kernel/security
tmpfs            15G  4.0K   15G   1% /dev/shm
devpts             0     0     0    - /dev/pts
tmpfs            15G   17M   15G   1% /run
tmpfs            15G     0   15G   0% /sys/fs/cgroup
cgroup             0     0     0    - /sys/fs/cgroup/systemd
pstore             0     0     0    - /sys/fs/pstore
cgroup             0     0     0    - /sys/fs/cgroup/cpuset
cgroup             0     0     0    - /sys/fs/cgroup/cpu,cpuacct
cgroup             0     0     0    - /sys/fs/cgroup/memory
cgroup             0     0     0    - /sys/fs/cgroup/devices
cgroup             0     0     0    - /sys/fs/cgroup/freezer
cgroup             0     0     0    - /sys/fs/cgroup/net_cls
cgroup             0     0     0    - /sys/fs/cgroup/blkio
cgroup             0     0     0    - /sys/fs/cgroup/perf_event
cgroup             0     0     0    - /sys/fs/cgroup/hugetlb
configfs           0     0     0    - /sys/kernel/config
/dev/xvda2      6.0G  5.5G  513M  92% /
selinuxfs          0     0     0    - /sys/fs/selinux
systemd-1          0     0     0    - /proc/sys/fs/binfmt_misc
debugfs            0     0     0    - /sys/kernel/debug
mqueue             0     0     0    - /dev/mqueue
hugetlbfs          0     0     0    - /dev/hugepages

So in the instructions it tells me I should store data in /var/lib/cassandra/data directory. But what am I going to do with the rest of the partitions ? I have enough space in other partitions but not much in rootfs. If I store in /var/lib/cassandra my rootfs become full in few minutes.

Can someone please help me, what am I missing here.