First, I know, I'm biased and 'like' RAID

This isn't an attempt to persuade anyone that RAID is the right answer for them - there's a lot it can't do that this type of approach can - effective use of mismatched drive sizes for instance.



For this issue, have you come across raid bitmaps?

They typically mean that a dirty shutdown even of an actively writing multi-TB raid will often be cleaned before it's even mounted. Yes you can add one to an existing RAID. Obviously they're not useful when a drive fails completely.

I also note that all data 'lost' when a drive dies under mhddfs is not available until the restore is done. Typically RAID provides zero downtime.

The real (and painful) risk of a second failure when a drive does fail means RAID6 or more highly redundant setups are often a better option if you really want to avoid downtime. I'm now using RAID6.

Interestingly you don't address the issue of the backup solution (a non-redundant cold spare?) failing as you read possibly many Tb of data from it? Isn't that the same problem as biting your nails whilst re-syncing an array with a new drive?