Comment by gregates

It really is important to understand the failure modes that the durability model accounts for and what it doesn't. It only accounts for "normal" failures, like an HDD reaching end of life.

For example, you mention Backblaze. Backblaze has public posts about their durability model. They claim to use 17:20 Reed-Solomon erasure encoding. That means there are 20 shards of a blob, and you can lose 3 of them and still reconstruct the blob.

Think about that for a second. If they store 4 shards in a datacenter, that means that a loss of that one datacenter is sufficient to lose the blob, forever. That entails that blobs are sharded across a minimum of 7 data centers, or the loss of one data center might mean permanent data loss. Which one do you think is true? (In fact it's pretty clear from Backblaze's public posts that they don't shard across data centers at all, only across racks within a data center.)

Now, AWS's availability guarantee — not their durability guarantee — entails that they use a less cost-effective erasure coding ratio. S3 is designed so that your blob is available even if a whole AZ goes down, and it's well known that most AWS regions have only 3 AZs. Therefore, if you tolerate the same number of shards lost to HDD failure as Backblaze in your durability model (3), then you might need 17:30 erasure coding to get the same durability and the required availability. That means S3 is storing way more physical bytes than Backblaze — 1.76x the logical size of the blob, instead of Backblaze's 1.18x. That's more expensive, but it also gives you better availability.

Which is also why One Zone S3 is cheaper — if you don't care about the availability guarantee, S3 can do what Backblaze does and save 33% on physical bytes, and they pass on 40–50% of those savings to the customer (this is fairer than it sounds — there's more overhead than physical storage bytes).

But here's the thing. AWS has more redundancy built in than Backblaze because they make availability guarantees in addition to durability guarantees. BUT the durability model is the same, which is why Backblaze can claim equivalent durability to S3. S3 in fact has better durability — they can survive the permanent loss of an AZ without necessarily losing blobs stored there (with the exception of One Zone blobs), and Backblaze cannot. But that's not actually a factor of the durability model, which is just taking into account normal events like HDD failure. Instead, S3 has durability that's more resilient to AZ loss because of their availability model. It's a side effect that isn't actually part of the durability promise!