Azure Managed Disk reservations allow organizations to pre-purchase Premium SSD capacity at a discounted rate by committing to a one-year term. However, these reservations operate on a strict use-it-or-lose-it basis — if reserved disk capacity is not matched by provisioned disks in a given hour, that hour's reservation benefit is permanently lost and does not carry forward. This means that any mismatch between reserved quantities and actual disk deployment directly erodes the value of the commitment. Organizations commonly encounter this waste when workloads are decommissioned, migrated to different disk SKUs, or moved to different regions after the reservation was purchased.

A critical nuance is that disk reservations are purchased by specific SKU (such as P30 or P40), not by aggregate capacity. A P40 reservation cannot be applied to P30 disk usage, even though both are Premium SSDs. This SKU-level rigidity creates a significant mismatch risk: if an organization resizes disks or shifts workloads to a different tier, the original reservation provides zero benefit. Combined with the relatively modest discount that disk reservations offer compared to other Azure reservation types, even a small amount of underutilization can quickly eliminate any savings and turn the reservation into a net cost increase.

The cost impact compounds over time. Because unused reservation hours are permanently lost, an organization paying for reservations that consistently go partially or fully unused is effectively paying more than it would under standard pay-as-you-go pricing — the worst possible outcome for a commitment designed to save money.