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AWS, Azure & GCP · Cloud across providers

What is block storage and how does it differ from object storage?

Mediumblock storageebsstorageUpdated Oct 2026
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Short answer

Block storage presents raw volumes that an instance formats with a filesystem and mounts as a disk. It offers low-latency random I/O for databases and boot volumes, but volumes are tied to one AZ.

A block volume behaves like a virtual disk. The guest sees a device, formats it, and reads and writes blocks. That makes it suitable for operating systems, databases and anything that needs a filesystem with random access.

How it differs

  • Block: low-latency random I/O, attached to one instance, tied to an AZ, snapshots for backup
  • Object: HTTP API, whole-object writes, highly durable and globally accessible, not mountable as a disk
  • File: shared POSIX filesystem over NFS or SMB, useful for shared content but with different latency
bash
sudo mkfs.ext4 /dev/nvme1n1
sudo mkdir -p /data
sudo mount /dev/nvme1n1 /data
df -h /data
Formatting and mounting a newly attached volume on Linux.

Volume type determines the performance envelope. General-purpose SSD volumes balance price and IOPS, provisioned IOPS volumes cost more for databases with strict latency, and throughput-optimised volumes suit large sequential reads. Snapshots are incremental and stored separately, but a snapshot is not a running replica.

A volume usually attaches to one instance at a time, and it lives in one AZ. For multi-AZ resilience, replicate at the application or database layer rather than assuming the disk follows the instance.

Follow-ups you should expect

Interviewers use these to check whether the first answer was understanding or memorisation.

  1. 1How do IOPS and throughput differ for block volumes?
  2. 2Why does a block volume not survive an AZ failure automatically?
  3. 3When would you choose a shared file system instead?
  4. 4How do snapshots differ from replication?

Say it in the interview

Block storage gives an instance a raw volume that it formats and mounts, which is what databases and boot disks need. It offers low-latency random I/O but is tied to an availability zone and usually to one instance. I use snapshots for backup and replicate at the database or application layer for resilience, rather than treating the disk as highly available by itself.

Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.

Related questions

EasyWhat is object storage and when should you use it?MediumWhen should you use a managed database?HardHow do you design backup and disaster recovery?

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