Reinstall os from scratch?

In 6.4.2 and later firmware that log file is now rotated so with port forwarding enabled you shouldn't run into a full root volume again from brute force SSH hack attempts. Personally I wouldn't recommend port forwarding but rather accessing SSH over a VPN if you must.

An OS Re-install wouldn't resolve a full root volume problem. It's useful in some specific situations and should only be run if we recommend it.

There are good reasons why wiping the OS and installing a clean copy is not documented for our users. It is only useful in certain extreme cases, requires an advanced level of technical expertise and if done improperly may put your data at risk as wiping the OS wipes all the config and the logs, some of which may well be critical to a successful data recovery attempt.

If you were to do a

# rm -rf /

You would be deleting your data as well as the OS. As to how much damage was done it would be a matter of when the system crashed as a result of trying to delete everything. We wouldn't be able to recover the data was lost.

Any ability to re-install the OS is no substitute for backing up your data. If your data is important to you then don't store it on just the one device, whatever that device may be.