Block Storage
also called Persistent Disk, EBS
A virtual disk attached to one instance at a time, presented as raw blocks and formatted with a filesystem — the storage databases and stateful workloads run on.
The properties that distinguish it from object storage: low latency, random read and write including partial updates, a filesystem, and attachment to a single instance (with some providers now offering limited multi-attach).
The characteristics to design around:
Provisioned capacity and IOPS. Unlike object storage, you buy a size and a performance level in advance. Under-provisioning IOPS is a common cause of unexplained database slowness — the volume is throttled, the database looks slow, and no application metric shows why.
Volume type matters more than size. General-purpose SSD, provisioned-IOPS SSD and throughput- optimised HDD have order-of-magnitude differences in latency and cost per IOPS. Some general-purpose volumes also accrue burst credits, with the same silent-throttling failure as burstable instances.
Zonal binding. A volume lives in one availability zone and cannot be attached across zones, which is why a zonal failure requires a snapshot restore rather than a reattachment, and why "multi-AZ" for stateful workloads means replication rather than shared disk.
Snapshots are incremental and stored in object storage, which makes them cheap and makes restore slower than people expect, because blocks are fetched lazily on first access.