> So I can go from an array of 3 x 10 TB disks where one is parity (20 TB usable storage), and then just pop in one more disk and now I have an array with 4 x 10 TB disks (30 TB usable storage) with the same one-disk parity. I can lose any one disk, and lose no data
RAID-5 is fragile. You can lose only one disk as you say, but the odds of succesful rebuild are not so great (assuming you have a NAS for data reliability in the first place).
> expand my storage at any time while keeping the same level of redundancy
But you don't keep the same level of redundancy when adding a drive. The more drives you add in RAID-5, the lower your probability of a successful rebuild after the loss of one drive.
I've seen a lot of articles and blog posts like this, but their numbers never seem to make sense. It says that reading through a 4-disk 8 TB array you only have a 15% chance of success. I have full-array BTRFS scrubbing scheduled monthly, according to this my array should have reported errors many times a year...
And of course, no matter what, no form of RAID/ZFS is a backup.
Based on those numbers and https://www.synology.com/en-us/support/RAID_calculator I'm guessing you're using RAID-5?
RAID-5 is fragile. You can lose only one disk as you say, but the odds of succesful rebuild are not so great (assuming you have a NAS for data reliability in the first place).
https://www.digistor.com.au/the-latest/Whether-RAID-5-is-sti...
> expand my storage at any time while keeping the same level of redundancy
But you don't keep the same level of redundancy when adding a drive. The more drives you add in RAID-5, the lower your probability of a successful rebuild after the loss of one drive.