Block Storage Snapshots Are Incremental Backups
A snapshot is an incremental, point-in-time backup of a disk volume, saving only changed data blocks. Use it for disaster recovery to restore a volume's exact state. The footgun: deleting an older snapshot may not save money if a newer one needs its data.
Why it exists
Raw disk volumes have no built-in history. If data is corrupted or accidentally deleted, it's gone forever. You need a mechanism to create backups for disaster recovery and data resiliency without the massive storage cost and time penalty of copying the entire disk every time.
The mental model
Think of a snapshot not as a full, independent copy, but as a set of instructions saying, "start with the previous snapshot, then apply these specific block changes." It's a chain of incremental diffs pointing back to an original full backup. The first snapshot is a full copy, but all subsequent ones only store what's new or modified.
How it works
When you trigger a snapshot, the system identifies which data blocks on the volume have changed since the most recent snapshot. Only these changed blocks are copied and stored, typically in a separate, highly durable object store like Amazon S3. Each snapshot contains all the information needed to restore the volume to its state at that point in time by reconstructing the data from the chain of incremental changes. When you delete a snapshot, only the data blocks that are unique to that snapshot are removed; any blocks still referenced by other snapshots are preserved.
When to use it
Use snapshots for regular backups of critical data volumes. They are essential for disaster recovery plans, allowing you to quickly launch a replacement volume from a known good state. They are also useful for creating copies of production volumes for use in testing or development environments.
When not to use it
Snapshots are not a replacement for live database replication. While you can snapshot a database volume, achieving transactional consistency often requires pausing I/O or using application-specific backup tools first. Snapshots are point-in-time, not continuous protection, and may not capture a database in a perfectly consistent state if writes are in-flight.
One canonical example
You have a 100GB volume. At 9 AM, you take Snapshot A. This copies all 100GB of data. At 10 AM, 5GB of data has changed. You take Snapshot B. This second snapshot only copies and stores that 5GB of changed data, referencing the other 95GB from Snapshot A. If you then delete Snapshot A, the 95GB of data it originally held is NOT deleted, because Snapshot B still depends on it.
Interview question
Which statement accurately describes the storage behavior of block storage snapshots?
- a.Subsequent snapshots only store the data blocks that have changed since the previous snapshot, referencing older data.Correct
- b.Each snapshot creates a complete, independent copy of the volume's data at that moment.
- c.Snapshots continuously synchronize data, providing real-time data protection and consistency.
- d.Deleting an older snapshot guarantees immediate and full recovery of the storage space it occupied.
Why? this is the answer
The card states that subsequent snapshots only store changed data blocks and reference older data from previous snapshots, making them incremental. This also means deleting an older snapshot does not necessarily free up all its space if newer snapshots still depend on its data, making option D incorrect.
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Read the original → docs.aws.amazon.com
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