Amazon EBS fast snapshot restore volume creation credits

The number of volumes that receive the full performance benefit of fast snapshot restore is determined by the volume creation credits for the snapshot. There is one credit bucket per snapshot per Availability Zone. Each volume that you create from a snapshot with fast snapshot restore enabled consumes one credit from the credit bucket. You must have at least one credit in the bucket to create an initialized volume from the snapshot. If you create a volume but there is less than one credit in the bucket, the volume is created without benefit of fast snapshot restore.

When you enable fast snapshot restore for a snapshot that is shared with you, you get a separate credit bucket for the shared snapshot in your account. If you create volumes from the shared snapshot, the credits are consumed from your credit bucket; they are not consumed from the snapshot owner's credit bucket.

The size of a credit bucket and the rate at which it refills depends on the size of the snapshot, not the size of the volumes created from the snapshot.

When you enable fast snapshot restore for a snapshot, the credit bucket starts with zero credits, and it gets filled at a set rate until it reaches its maximum credit capacity. Also, as you consume credits, the credit bucket is refilled over time until it reaches its maximum credit capacity.

The fill rate for a credit bucket is calculated as follows:

MIN (10, (1024 ÷ snapshot_size_gib))

And the size of the credit bucket is calculated as follows:

MAX (1, MIN (10, (1024 ÷ snapshot_size_gib)))

For example, if you enable fast snapshot restore for a snapshot with a size of 128 GiB, the fill rate is 0.1333 credits per minute.

MIN (10, (1024 ÷ 128))
 = MIN (10, 8)        
 = 8 credits per hour
 = 0.1333 credits per minute

And the maximum size of the credit bucket is 8 credits.

MAX (1, MIN (10, (1024 ÷ 128)))      
 = MAX (1, MIN (10, 8))
 = MAX (1, 8)
 = 8 credits

In this example, when you enable fast snapshot restore, the credit bucket starts with zero credits. After 8 minutes, the credit bucket has enough credits to create one initialized volume (0.1333 credits × 8 minutes = 1.066 credits). When the credit bucket is full, you can create 8 initialized volumes simultaneously (8 credits). When the bucket is below its maximum capacity, it refills with 0.1333 credits per minute.

You can use CloudWatch metrics to monitor the size of your credit buckets and the number of credits available in each bucket. For more information, see Metrics for fast snapshot restore.

After you create a volume from a snapshot with fast snapshot restore enabled, you can describe the volume using describe-volumes and check the fastRestored field in the output to determine whether the volume was created as an initialized volume using fast snapshot restore.