Understanding Backup Storage Usage

Amazon DocumentDB backup storage consists of continuous backups within the backup retention period and manual snapshots outside the retention period. To control your backup storage usage, you can reduce the backup retention interval, remove old manual snapshots when they are no longer needed, or both. For general information about Amazon DocumentDB backups, see Backing Up and Restoring in Amazon DocumentDB. For pricing information about Amazon DocumentDB backup storage, see Amazon DocumentDB Pricing.

To control your costs, you can monitor the amount of storage consumed by continuous backups and manual snapshots that persist beyond the retention period. Then you can reduce the backup retention interval and remove manual snapshots when they are no longer needed.

You can use the Amazon CloudWatch metrics TotalBackupStorageBilled, SnapshotStorageUsed, and BackupRetentionPeriodStorageUsed to review and monitor the amount of storage used by your Amazon DocumentDB backups, as follows:

• BackupRetentionPeriodStorageUsed represents the amount of backup storage used for storing continuous backups at the current time. This metric value depends on the size of the cluster volume and the number of changes you make during the retention period. However, for billing purposes the metric does not exceed the cumulative cluster volume size during the retention period. For example, if your cluster size is 100 GiB and your retention period is two days, the maximum value for BackRetentionPeriodStorageUsed is 200 GiB (100 GiB + 100 GiB).

• SnapshotStorageUsed represents the amount of backup storage used for storing manual snapshots beyond the backup retention period. Manual snapshots taken within the retention period do not count against your backup storage. Similarly, automatic snapshots do not count against your backup storage. The size of each snapshot is the size of the cluster volume at the time you take the snapshot. The SnapshotStorageUsed value depends on the number of snapshots you keep and the size of each snapshot. For example, suppose that you have one snapshot outside the retention period and cluster volume size was 100 GiB when that snapshot was taken. The amount of SnapshotStorageUsed is 100 GiB.

• TotalBackupStorageBilled represents the sum of BackupRetentionPeriodStorageUsed and SnapshotStorageUsed, minus an amount of free backup storage equal to the size of cluster volume for one day. For example, if your cluster size is 100 GiB, you have one retention day, and you have one snapshot outside the retention period, the TotalBackupStorageBilled is 100 GiB (100 GiB + 100 GiB - 100 GiB).

• These metrics are computed independently for each Amazon DocumentDB cluster.

You can monitor your Amazon DocumentDB clusters and build reports using CloudWatch metrics through the CloudWatch console. For more information about how to use CloudWatch metrics, see Monitoring Amazon DocumentDB.