Como mencionado em Como posso ver meus créditos de estouro de IOPS? ( source ):
Unfortunately there isn't a Cloudwatch Metric for the current IOPS Credit balance like there is with the CPU credits.
Isso ainda não foi implementado em julho de 2016.
Os documentos da AWS oferecem uma boa introdução ao burst IOPS:
Each volume receives an initial I/O credit balance of 5.4 million 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 at the baseline performance rate of 3 IOPS per GiB of volume size. For example, a 100 GiB gp2 volume has a baseline performance of 300 IOPS.
When your volume requires more than the baseline performance I/O level, it draws on 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 baseline performance that is equal or greater than the maximum burst performance, and their I/O credit balance never depletes. 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 (5.4 million I/O credits).
Então, tomando o seu exemplo, seu volume de 20 GB tem um desempenho de linha de base de 60 IOPS. Se você usar uma média de 50 IOPS, 10 créditos de I / O serão adicionados a seu saldo de crédito a cada segundo, até um máximo de 5,4 milhões de créditos de I / O.