The Cloud in 2018: what we have learned so far
What does 11 9s durability really mean?
Marketing's hype frontier is the cloud. Cloud vendors need trust. So they claim 99.999999999 percent availability. Huh? Why not 15 or lebenty lebben 9s? Here's the deal.
The good folks at Backblaze published a post[1] today on the truth behind cloud durability claims. If you want the gory details, go read the whole thing.
If not, keep reading.
Forget the math
The math behind availability numbers is impressive. But the math depends on the assumptions behind it.
The assumptions are key. And they are:
- Average rebuild time. Which isn't what you think it is.
- Annualized drive failure rate, or AFR. Also not what you think.
All very scientific, except they're beside the point.
Why?
Modern cloud storage is a tech wonder. Active cloud storage typically has three copies of your data. Partly for availability, but also for performance, since 7200 RPM disks have more than 8ms of rotational latency, and with three you cut that number significantly.
Backup storage - what Backblaze offers - can't afford three copies, so they use fancy erasure codes to protect data. These systems break data into shards with some mathematically intense parity that typically enables data to survive four drive failures with little impact on storage capacity.
Vendors don't wait until four drives fail to take corrective action. The shards are spread over the infrastructure so no single power supply, switch, or rack can take out more than one shard. And rebuilds are usually highly parallel, so the failures are repaired a lot faster than a home user would expect.
The point?
As the Backblaze post notes, 11 9s