What is the difference between durability and availability for storage?
What they are really testing: Whether you understand these are two distinct guarantees, durability is not losing data, availability is being able to access it now, and that data can be durable but temporarily unavailable.
A real interview question
What is the difference between durability and availability for storage?
What most people say
drag me
“They both mean how reliable your storage is.”
Conflating them is the mistake. Durability is about not losing data (eleven nines); availability is about being able to reach it now (often three nines). They are different guarantees with different numbers and mechanisms.
The follow-ups they ask next
Can data be durable but unavailable? Give an example.
Yes, a service outage makes objects unreadable for a while, but the bytes are intact and access returns. Durability (data not lost) and availability (data reachable now) are independent guarantees.
How is eleven-nines durability actually achieved?
By storing redundant copies (or erasure-coded shards) across multiple devices and availability zones, plus checksums to detect and repair corruption, so no single failure, or even several, loses the data.
What the interviewer is listening for
- Durability = not lost, availability = reachable now
- Knows the different magnitudes (11 nines vs ~3)
- Knows data can be durable but unavailable
What sinks the answer
- Conflates the two
- No notion of the different numbers
- Thinks unavailable means lost
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“Durability is [the probability data is not lost over time, object stores quote ~11 nines via redundancy]. Availability is [the probability you can access it right now, often ~3 nines]. They are [independent: data can be durable but temporarily unavailable, an outage, nothing lost]. Durability comes from [redundant copies across failure domains]; availability from [redundant serving paths + failover].”
Keep going with storage
Foundation
What is the difference between backup and replication, and why is replication not a backup?
Junior
Explain the difference between object, block, and file storage, and give one good use case for each.
Junior
How does object storage achieve such high durability?
Junior
What consistency guarantees does object storage give, and why does it matter?
Junior
For block storage, what is the difference between IOPS and throughput, and how does it affect volume choice?
Mid
Design durable, highly available storage for user-uploaded images. Which service do you choose and why?
Knowing the answer is not the same as recalling it under pressure
Sign in to send the questions you fumble to spaced recall, so they come back right before you would forget them, and learn the concepts behind them with hands-on labs.
Start free