In late 2023 we live in an era of super cheap storage. Be it flash based or spinning rust, they can be regularly had for 30€ and 13€ per TB respectively. Just a few years ago you had pick very specific high density (for the time) disks to get under 20€ per TB. This isn’t necessarily practical if you only need moderate amounts of storage but want good redundancy. The true budget option is used magnetic storage, going for well under 3€ per TB. But how can you safely make use of worn, old disks? That’s where ZFS comes into play.
Used HDD pricing in 2023
The idea of using smaller used disks with high redundancy comes from the dramatic savings that can be achieved. I have seen several lots of 10-40 disks priced as low as 2.50€/TB. Now, using this many disks at once is somewhat impractical, even with high redundancy and (hot-)spares. But ironically, selling a part of such a lot as individual disks will increase their value, since most people don’t want the hassle of buying this many disks. Although individual disks (of this specific type) can go for similarly low prices, they can also go for up to 15€. Either way you would want to keep some spares.
Reliability and redundancy with ZFS
Now the reason why people are reluctant to buy used HDDs is due their limited lifetimes. Being mechanical in nature they will die eventually. Especially these super cheap lots of enterprise disks will have had a hard life with extremely high runtime in data centers. However:
- These lots usually advertise their disks as 100% health.
- They are enterprise grade and designed for very high uptime and high reliability
- HDDs have come a long way. Even these older models are likely to be much more reliable than the ones that caused HDDs’ reputation for high failure rates.
With that being said you will want to plan for failing disks, data corruption and bit rot. You should do that for any kind of storage, but for used Hard Drives especially. And the best system that gives you both redundancy and corruption resilience at home is ZFS. ZFS is a file system with native support for RAID, or replication of data across multiple disks through parity data. Unlike traditional RAID systems, however it also provides resiliency against bit rot.
Bit rot is the degradation and corruption of data that can occur on many media. It can have many causes, but basically this phenomenon means that any storage media over time will lose and corrupt data and the only protection against that is parity and a system that can repair data from that parity. Traditional RAID will protect against failing hard disks, but it has no way of knowing which bits may have rotted or not. ZFS does that.
How does it work?
There are already many good ZFS tutorials out there, but to keep it theoretical, for very good resilience with used disks you may want to use RAIDZ3, meaning having the capacity of 3 disks as redundancy. If you had two four-port SAS-controllers (which can be had for well under 5€ each) you might have eight disks connected and keep two as cold spares. You could sell off the rest of the lot or keep it for future expansion. The ZFS calculator suggests you would then have 12TB of usable capacity, with cold spares and very high reliability for under 40€ all in. Sounds like a good deal to me.