DRBD is sane, but of course not a backup. If the splunk system hardware has a memory failure and scribbles nonsense into the database, DRBD will faithfully mirror the nonsense.
The problem with backups is they work file-at-a-time so they do not get a time-coherent dataset. This limitation does not apply to DRBD, which operates at the block layer.
DRBD is a good tool, and there is no reason splunk cannot work with it (some customers have done so). The main caveat is that there are some scenarios where splunk may not be happy with the dataset it is given on startup, and may not automatically recover from the provided data. This is true with most systems, but historically Splunk has had trouble more often than a less data-intensive tool like, say, apache. However, significant improvments have been made in data error recovery and avoidance.
If DRBD seems appropriate for you, then by all means use it, but it certainly raises the administration effort involved. I would definitely recommend evaluating combining this with a backup strategy (as above). On Linux, I would evalute LVM2 for snapshotting-to-backup goals.
