That number is probably the estimated probability that over the course of a year, a single vault loses every file contained on it (which is what would happen if a single vault had 4 drives simultaneously and irrecoverably fail).
As a consumer, the type of failure you would experience, and the probability of experiencing that failure, given that Backblaze has suffered a vault failure, depends on how they distribute your data amongst their vaults. They don't explicitly say how they do this, so it's impossible to know for sure, but we can consider the two extreme scenarios.
Scenario 1: Each customer is assigned a single vault, and all your files are on it. In this case, if Backblaze lost a vault, you would either luck out and have your files on another vault and be completely unaffected, or get really screwed and have all your files on the bad vault, and lose them all. They've got 150 PB of storage, and each vault stores 3.6 PB of data, so we can estimate that currently you may have something like a 1 in 40 chance of having your data on any given vault. So under this scenario, you would have a 1 in 400 million chance of losing all your files.
Scenario 2: Each customer's files are uniformly distributed across all vaults. In this case, if Backblaze lost a vault, all customers would lose a fraction of their files. Again, using our estimate that they might have 40 vaults, you would have a 1 in 10 million chance of losing 2.5% of your files.
So up to now, we're basically just doing the math without questioning the assumptions of the model. In reality, I think your practical risk is mostly concentrated in things outside of the model: ie, an event that affects all of their vaults simultaneously, like a fire, earthquake, meteor strike, etc. If I had to make a bet about what that number is, I'd put it in the 1/10,000 to 1/100,000 range. In other words, orders of magnitude higher than losing data because some hard drives failed, or a backblaze employee spilled his coffee, or something like that.
Thanks. IMHO the greatest risk of data loss is bugs in the homegrown software and/or operator error during maintenance operations, not a natural disaster. We infallible software engineers always underestimate that stuff, but it's usually the cause.
Also, I'm not worried. If that probability only concerns data loss on Backblaze's side, even if it's 1/10,000, then that's still not the probability of actual customer data loss. Because for that to happen there'd have to be a simultaneous loss of data on the customer side as well. That probably extends the durability considerably.
Brian from Backblaze here - expanding on Yev's answer a little, a customer has an email address/password that they login to Backblaze's datacenter with. The email address is bound to a Backblaze "Cluster" for life (at least so far we have never migrated a customer between clusters). The cluster contains a variety of services, it's the unit of scaling for our company. A single cluster scales easily up to at least 200 Vaults (probably much more, we'll let you know), and any one customer's data will be spread across pretty much all the Vaults in that cluster with enough time. A customer backs up once per hour, and each time they are told to backup to a vault with spare space on it with no affinity. Theoretically it might be possible all your data lands on one vault, but it's really unlikely.
Vaults belong to a cluster - so your backup only puts data on the group of Vaults assigned to that one cluster, and the vaults in that cluster don't contain data from customers on OTHER clusters.
Yev from Backblaze -> Customers/files are striped across multiple vaults for added security, so odds of all data from a single user going poof are fairly high and would most likely come from the datacenter falling in to a surprise sinkhole.
As a consumer, the type of failure you would experience, and the probability of experiencing that failure, given that Backblaze has suffered a vault failure, depends on how they distribute your data amongst their vaults. They don't explicitly say how they do this, so it's impossible to know for sure, but we can consider the two extreme scenarios.
Scenario 1: Each customer is assigned a single vault, and all your files are on it. In this case, if Backblaze lost a vault, you would either luck out and have your files on another vault and be completely unaffected, or get really screwed and have all your files on the bad vault, and lose them all. They've got 150 PB of storage, and each vault stores 3.6 PB of data, so we can estimate that currently you may have something like a 1 in 40 chance of having your data on any given vault. So under this scenario, you would have a 1 in 400 million chance of losing all your files.
Scenario 2: Each customer's files are uniformly distributed across all vaults. In this case, if Backblaze lost a vault, all customers would lose a fraction of their files. Again, using our estimate that they might have 40 vaults, you would have a 1 in 10 million chance of losing 2.5% of your files.
So up to now, we're basically just doing the math without questioning the assumptions of the model. In reality, I think your practical risk is mostly concentrated in things outside of the model: ie, an event that affects all of their vaults simultaneously, like a fire, earthquake, meteor strike, etc. If I had to make a bet about what that number is, I'd put it in the 1/10,000 to 1/100,000 range. In other words, orders of magnitude higher than losing data because some hard drives failed, or a backblaze employee spilled his coffee, or something like that.