Data allocation strategies for the management of Quality of Service in Virtualised Storage Systems

The amount of data managed by organisations continues to grow relentlessly. Driven by the high costs of maintaining multiple local storage systems, there is a well established trend towards storage consolidation using multi-tier Virtualised Storage Systems (VSSs). At the same time, storage infrastructures are increasingly subject to stringent Quality of Service (QoS) demands. Within a VSS, it is challenging to match desired QoS with delivered QoS, considering the latter can vary dramatically both across and within tiers. Manual efforts to achieve this match require extensive and ongoing human intervention. This paper presents our work on the design and implementation of data allocation strategies in an enhanced version of the popular Linux Extended 3 Filesystem. This enhanced fllesystem features support for the specification of QoS metadata while maintaining compatibility with stock kernels. We present new inode and datablock allocation strategies which seek to match the QoS attributes set by users and/or applications on files and directories with the QoS actually delivered by each of the filesystem's block groups. To create realistic test filesystems we have modified the Impressions benchmarking framework to support QoS metadata. The effectiveness of the resulting data allocation in terms of QoS matching is evaluated using a special kernel module that is capable of inspecting detailed filesystem allocation data on the-fly. We show that our implementations of the proposed inode and datablock allocation strategies are capable of dramatically improving data placement with respect to QoS requirements when compared to the default allocators.