Delay Performance of Direct Reads in Distributed Storage Systems with Coding

Delay performance is crucial in distributed storage systems and it can greatly impact user experience. Both direct and k-access reads are common in storage systems. However, much of previous research only considers k-access reads and many schemes, such as Redundant Scheme, are only shown to reduce delay for k-access reads. We have no idea whether those existing schemes can also work for direct reads. The study regarding the characteristics of the delay performance of direct reads, and the appropriate schemes for direct reads to reduce delay is still lacking. In this paper, we study the delay performance of direct reads and its correlation with degraded reads. We illustrate the relationship between degraded reads and bandwidth cost and answer important questions like when degraded reads can help reduce delay. Then we propose a scheme DRALB to reduce delay for direct reads. DRALB can be easily added to existing schemes and can greatly reduce the delay of hot data. We also conduct trace-driven simulations to verify that DRALB significantly outperforms existing schemes, in terms of delay performance of direct reads.