When I launch an Amazon MySQL database instance in RDS, I choose the amount of Allocated Storage for it.
When I create a snapshot (either manually or with the automatic backup), it says under “Storage” the same size as the size allocated for the instance, even though my database did not reach that size.
Since the pricing (or the free tier) in Amazon is dependent on the amount of storage used, I would like to know the real storage size I’m using, rather than the size allocated by the original database.
From looking at the Account Activity, and from knowing how mysqldump works, I would guess the snapshot does not really include the empty space allocated.
RDS is being stored through EBS according to FAQ:
Amazon RDS uses EBS volumes for database and log storage.
EBS doesn’t store empty blocks, according to its pricing page:
Because data is compressed before being saved to Amazon S3, and Amazon EBS does not save empty blocks, it is likely that the snapshot size will be considerably less than your volume size.
And takes space only for changed blocks after initial snapshot was made, according to details page:
If you have a device with 100 GB of data but only 5 GB has changed after your last snapshot, a subsequent snapshot consumes only 5 additional GB and you are billed only for the additional 5 GB of snapshot storage, even though both the earlier and later snapshots appear complete.