How Amazon EBS snapshots work - Amazon EBS
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How Amazon EBS snapshots work

The first snapshot that you create from a volume is always a full snapshot. It includes all of the data blocks written to the volume at the time of creating the snapshot. Subsequent snapshots of the same volume are incremental snapshots. They include only changed and new data blocks written to the volume since the last snapshot was created

The size of a full snapshot is determined by the size of the data being backed up, not the size of the source volume. Similarly, the storage costs associated with a full snapshot is determined by the size of the snapshot, not the size of the source volume. For example, you create the first snapshot of a 200 GiB Amazon EBS volume that contains only 50 GiB of data. This results in a full snapshot that is 50 GiB in size, and you are billed for 50 GiB snapshot storage.

Similarly, the size and storage costs of an incremental snapshot are determined by the size of any data that was written to the volume since the previous snapshot was created. Continuing the previous example, if you create a second snapshot of the same 200 GiB volume after changing 20 GiB of data and adding 10 GiB of data, the incremental snapshot is 30 GiB in size. You are then billed for that additional 30 GiB snapshot storage.

For more information about snapshot pricing, see Amazon EBS pricing.

Important

When you archive an incremental snapshot, it is converted to a full snapshot that includes all of the blocks written to the volume at the time that the snapshot was created. It is then moved to the Amazon EBS Snapshots Archive tier. Snapshots in the archive tier are billed at a different rate from snapshots in the standard tier. For more information, see Pricing and billing for archiving Amazon EBS snapshots.

The following sections show how an EBS snapshot captures the state of a volume at a point in time, and how subsequent snapshots of a changing volume create a history of those changes.

Multiple snapshots of the same volume

The diagram in this section shows Volume 1, which is 15 GiB in size, at three points in time. A snapshot is taken of each of these three volume states. The diagram specifically shows the following:

  • In State 1, the volume has 10 GiB of data. Snap A is the first snapshot taken of the volume. Snap A is a full snapshot and the entire 10 GiB of data is backed up.

  • In State 2, the volume still contains 10 GiB of data, but only 4 GiB have changed after Snap A was taken. Snap B is an incremental snapshot. It needs to back up only the 4 GiB that changed. The other 6 GiB of unchanged data, which are already backed up in Snap A, are referenced by Snap B rather than being backed up again. This is indicated by the dashed arrow.

  • In State 3, 2 GiB of data have been added to the volume, for a total of 12 GiB, after Snap B was taken. Snap C is an incremental snapshot. It needs to back up only the 2 GiB that were added after Snap B was taken. As shown by the dashed arrows, Snap C also references the 4 GiB of data stored in Snap B, and the 6 GiB of data stored in Snap A.

  • The total storage required for the three snapshots is 16 GiB total. This accounts for 10 GiB for Snap A, 4 GiB for Snap B, and 2 GiB for Snap C.

Snapshots capturing an initial volume state and two subsequent states after data has been changed.

Incremental snapshots of different volumes

The diagram in this section shows how incremental snapshots can be taken from different volumes.

  1. Vol 1, which is 14 GiB in size, has 10 GiB of data. Because Snap A is the first snapshot taken of the volume, it is a full snapshot and the entire 10 GiB of data is backed up.

  2. Vol 2 is created from Snap A, so it is an exact replica of Vol 1 at the time the snapshot was taken.

  3. Over time, 4 GiB of data is added to Vol 2 and the total size of its data is 14 GiB.

  4. Snap B is taken from Vol 2. For Snap B, only the 4 GiB of data that was added after the volume was created from Snap A is backed up. The other 10 GiB of unchanged data, which is already stored in Snap A, is referenced by Snap B instead of being backed up again.

    Snap B is an incremental snapshot of Snap A, even though it was created from a different volume.

Important

The diagram assumes that you own Vol 1 and Snap A, and that Vol 2 is encrypted with the same KMS key as Vol 1. If Vol 1 was owned by another Amazon account and that account took Snap A and shared it with you, then Snap B would be a full snapshot. Or, if Vol 2 was encrypted with a different KMS key than Vol 1, then Snap B would be a full snapshot.

Snapshots capturing an initial volume state and two subsequent states after data has been changed.

For more information about how data is managed when you delete a snapshot, see Delete an Amazon EBS snapshot.