My client has a 12-bay Synology DiskStation DS3617xs NAS filled with 14TB Seagate IronWolf Pros. It will be used in an adjacent building with Active Backup for Business to backup the server and 40 workstations.
Sadly, according to support, triple parity isn't an option with Synology, so I'm trying to decide whether it's reasonably safe to go with RAID6 or if the only logical choice is RAID10.
I've understood for a while that the size of modern hard drives makes RAID5 a bad idea due to the likelihood of an unrecoverable read error during a rebuild. I've seen a small amount of discussion on whether RAID6 is a good idea, but haven't come across any details other than rebuild time. I have no first-hand experience for what kind of duration to expect when rebuilding a 140TB array.
My own take on whether RAID6 is reasonably safe is that a rebuild would only fail with a single drive failure if two of the remaining drives both had UREs on sectors that contain the exact same data. Granted, if you have two concurrent drive failures, you're in the same boat as with one drive failure on RAID5, nervously biting your nails and praying that the rebuild finishes.
Is my understanding of the minimal risk involved with UREs and RAID6 accurate? How much time is it likely to take to rebuild after a drive failure?