Q:

Amazon EC2 gives no disk space but my instance has space

So i have an instance that supposedly has a lot of free space. 
I have installed Postgres and when i try to import a dump it tells me my disk is full. I did a df -h and saw that in fact xvda1 is full, but what about `xvdb ?
it has a lot of free space, how can i use that for my Postgres database??
df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1      7.9G  7.5G  1.1M 100% /
udev            1.9G  8.0K  1.9G   1% /dev
tmpfs           376M  184K  375M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            1.9G     0  1.9G   0% /run/shm
/dev/xvdb       394G  199M  374G   1% /mnt

A:

xvdb is most likely an the Amazon EC2 Instance Store volume (also called an ephemeral volume, in contrast to an Amazon EBS volume) and you almost certainly do not want to use it if you value your data, except if you exactly know what you are doing and are prepared to always have point in time backups etc.:

Ephemeral storage will be lost on stop/start cycles and can
  generally go away, so you definitely don't want to put anything of
  lasting value there, i.e. only put temporary data there you can
  afford to lose or rebuild easily, like a swap file or strictly
  temporary data in use during computations. Of course you might store
  huge indexes there for example, but must be prepared to rebuild these
  after the storage has been cleared for whatever reason (instance
  reboot, hardware failure, ...).

See my answers to the following related questions for more details:

Will moving data from EBS to ephemeral storage improve MySQL query performance?
how to take backup of aws ec2 instance/ephemeral storage?

