Amazon EBS Volume Types

General Purpose (SSD) volume performance is governed by volume size, which dictates the base performance level of the volume and how quickly it accumulates I/O credits; larger volumes have higher base performance levels and accumulate I/O credits faster. I/O credits represent the available bandwidth that your General Purpose (SSD) volume can use to burst large amounts of I/O when more than the base performance is needed. The more credits your volume has for I/O, the more time it can burst beyond its base performance level and the better it performs when more performance is needed.

Each volume receives an initial I/O credit balance of 5,400,000 I/O credits, which is enough to sustain the maximum burst performance of 3,000 IOPS for 30 minutes. This initial credit balance is designed to provide a fast initial boot cycle for boot volumes and to provide a good bootstrapping experience for other applications. Volumes earn I/O credits every second at a base performance rate of 3 IOPS per GiB of volume size. For example, a 100 GiB General Purpose (SSD) volume has a base performance of 300 IOPS.

When your volume requires more than the base performance I/O level, it simply uses I/O credits in the credit balance to burst to the required performance level, up to a maximum of 3,000 IOPS. Volumes larger than 1,000 GiB have a base performance that is equal or greater than the maximum burst performance, so their I/O credit balance never depletes and they can burst indefinitely. When your volume uses fewer I/O credits than it earns in a second, unused I/O credits are added to the I/O credit balance. The maximum I/O credit balance for a volume is equal to the initial credit balance (5,400,000 I/O credits).

If your volume uses all of its I/O credit balance, the maximum performance of the volume will remain at the base performance level (the rate at which your volume earns credits) until I/O demand drops below the base level and unused credits are added to the I/O credit balance. The larger a volume is, the greater the base performance is and the faster it replenishes the credit balance.

The table below lists several volume sizes and the associated base performance of the volume (which is also the rate at which it accumulates I/O credits), the burst duration at the 3,000 IOPS maximum, and the time in seconds that the volume would take to refill an empty credit balance.

The burst duration of a volume is dependent on the size of the volume, the burst IOPS required, and the credit balance when the burst begins. This is shown in the equation below:

Burst duration = (Credit balance)/((Burst IOPS) - 3(Volume size in GiB))

If you notice that your volume performance is frequently limited to the base level (due to and empty I/O credit balance), you should consider using a larger General Purpose (SSD) volume (with a higher base performance level) or switching to a Provisioned IOPS (SSD) volume for workloads that require sustained IOPS performance greater than 3,000 IOPS.