| > I don't understand why you'd want to I want to add a single drive since I can't afford more than a single drive. But I still want to keep the data security of one or more parity drives. Synology lets me do that. ZFS doesn't. On a Synology NAS (which just uses Linux mdraid underneath the hood so this part isn't exactly some proprietary magic) if you have an array with parity (the equivalent of raid-z/z2), you can add a drive, and it expands the array with that one drive, keeping the parity and recalculating it for the new configuration of drives. 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. ZFS can't do that, since it does't support modifying vdevs. So if I want to be able to add a single drive and expand my storage at any time while keeping the same level of redundancy, ZFS makes no sense. Synology's configuration of mdraid+BTRFS makes way more sense than ZFS. Unfortunately they haven't contributed it to free software so nobody else can have it (specifically the part of passing through the parity data so that checksum errors in BTRFS can be fixed with mdraid knowledge). I would prefer to not have to rely on Synology's cost-cutting hardware and raft of probably not very secure software. But for the use case of me and the small businesses I support, ZFS has been a non-starter due to the costs. |
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.