# Amazon Neptune storage, reliability and availability Amazon Neptune uses a distributed and shared storage architecture that scales automatically as your database storage needs grow. Neptune data is stored in a cluster volume, which is a single, virtual volume that uses Non-Volatile Memory Express (NVMe) SSD-based drives. A cluster volume can grow to a maximum size of 128 tebibytes (TiB) (64 TiB in the China Regions and the AWS GovCloud (US) Regions). A cluster volume consists of copies of the data across three Availability Zones (AZs) in a single AWS Region, which provides high durability and availability. The [Storage](storage.md) section covers these topics in detail: + **[Allocation](storage.md#storage-allocation)** – You are charged for space allocated, as determined by the storage high water mark. + **[Storage billing](storage.md#storage-billing)** – Storage costs are billed based on the high water mark, and Neptune also offers an [I/O–Optimized](storage-types.md#provisioned-iops-storage) pricing option for I/O–intensive workloads. + **[Reliability and high availability](storage.md#storage-reliability)** – Neptune replicates your data across three Availability Zones for high durability and automatic fault recovery.