Understanding the Resiliency of Cloud Storage Services

A cloud storage system requires multiple functional and management layers to render a global-scale storage solution. Providing a reliable service through this complex architecture becomes a challenging task. Moreover, the mere complexity of the system makes it difficult to identify the critical components for maintaining a reliable service. While data redundancy has been the predominant factor in improving reliability, it becomes crucial to understand if it is sufficient for the reliability of a cloud storage service. This work proposes a resiliency evaluation method that identifies the components necessary for storage service rendition and the ability of the system to absorb the effect of their failures and minimize the impact on service. Using this method, the resiliency of two distinct cloud storage services, OpenStack Swift and CephFS, are evaluated. The evaluation has revealed that data access requests may get delayed (up to 8x of mean response time) and even fail due to the lack of resiliency for access path components, even when there is enough user data redundancy. The lack of effort is evident while maintaining the consistency of critical internal data components resulting in reduced reliability. Even for stored data, some common failures result in loss of recoverability. The work concludes with some general observations and possible solutions that will help to improve storage service reliability.