In the literature about RAID configurations, a so-called Hot Spare is recommended for RAID 5 configurations in the event two drives go out at the same time. Synology has a set up for a Hot Spare on its Storage Page. Is this something you recommend for a NAS with 3 or more drives?
Like most technical decisions, it depends…
- RAID 5 + hot spare is not appreciably more reliable than RAID 5. It minimizes the delay in starting a rebuild but can’t survive a second failure during the rebuild. Basically, it saves you the time it takes to acquire and install a cold spare.
- If you can afford the extra disk then RAID 6 is more reliable, albeit at the expense of write performance.
- A 1-2-3 backup strategy is important in either case
There are several RAID reliability calculators like this one on wintelguy.com that can help you make a decision.
FWIW, I have a 4 disk RAID 5 configuration for my needs. RAID 60 would be a consideration if I had dozens or hundreds of disks.