Technically you do have to pre-allocate, but it works like pay-on-demand because:
<paste>
We can increase the size of your filesystem at any time, with no downtime involved - so there is no need to overbuy in anticipation of future usage.
Further, you are granted a +10% grace at all times, so you always have some room to grow.
Finally, the system automatically emails you as you get close to your limit, so there is never an unexpected filling of space.
</paste>
Technically you do have to pre-allocate, but it works like pay-on-demand because:
<paste>
We can increase the size of your filesystem at any time, with no downtime involved - so there is no need to overbuy in anticipation of future usage.
Further, you are granted a +10% grace at all times, so you always have some room to grow.
Finally, the system automatically emails you as you get close to your limit, so there is never an unexpected filling of space.
</paste>