Ephemeral Storage

Ephemeral storage is defined by its retention guarantees, not by a particular disk technology or storage interface. It can be backed by local SSDs, other disks, or memory, depending on the platform. The important question is whether data remains available when the associated compute environment stops, is removed, fails, or is replaced. Applications should treat data on ephemeral storage as expendable unless a separate mechanism preserves it.

In cloud architectures, ephemeral storage provides working space close to application execution. A video-processing worker might download a source object, create temporary frames on its local disk, and upload the completed video to durable object storage. A build worker might use temporary storage for source checkouts and compilation artifacts. If either worker disappears, another worker can repeat the operation using the original inputs.

AWS exam questions commonly test this concept through the distinction between Amazon EC2 instance store and Amazon Elastic Block Store, or Amazon EBS. Instance store provides temporary block-level storage on disks physically attached to the host computer. It is available on supported EC2 instance types and is suitable for buffers, caches, scratch files, and reproducible intermediate results. Its data survives an ordinary instance reboot, but is lost when the instance is stopped, hibernated, or terminated, or when the underlying disk fails. Rebooting and stopping an instance are therefore not equivalent storage lifecycle events.

Amazon EBS provides persistent block storage with a lifecycle that can be managed separately from the instance. An EBS volume retains its data when its attached instance stops. Whether a volume is deleted when the instance terminates depends on its delete-on-termination setting; root volumes are commonly configured for deletion by default. Consequently, persistent does not mean impossible to delete, and an EBS volume used for temporary files does not become instance store. Both EBS and instance store provide block storage, but their persistence guarantees differ.

Ephemeral storage also appears in container and serverless platforms. A container's writable layer should not be treated as durable application storage. Function environments may retain temporary files between invocations when an environment is reused, but reuse is not guaranteed. Exact retention boundaries vary by platform, so applications must not assume that all temporary storage disappears after every request or survives every restart.

The main trade-off is convenient working storage versus responsibility for recovery. Host-local storage can provide low-latency access and avoid network storage traffic, but capacity and performance depend on the compute configuration. Ephemeral storage is not necessarily memory-backed, universally faster, or free. Applications must also monitor free space, remove obsolete temporary files, and handle out-of-space errors.

A reliable design stores authoritative inputs and completed outputs in durable storage, makes interrupted work safe to retry, and treats local caches as optional accelerators. Logs needed for troubleshooting should be exported rather than left only on a disposable disk. Replication or checkpointing can protect a workload using ephemeral disks, but those protections come from the surrounding architecture, not from the ephemeral storage itself.