Optimization and Bottleneck Analysis of Network Block I/O in Commodity Storage Systems

Building commodity networked storage systems is an important architectural trend; Commodity servers hosting a moderate number of consumer-grade disks and interconnected with a high-performance network are an attractive option for improving storage system scalability and cost-efficiency. However, such systems incur significant overheads and are not able to deliver to applications the available throughput. We examine in detail the sources of overheads in such systems, using a working prototype to quantify the overheads associated with various parts of the I/O protocol. We optimize our base protocol to deal with small requests by batching them at the network level and without any I/O-specific knowledge. We also redesign our protocol stack to allow for asynchronous event processing, in-line, during send-path request processing. These techniques improve performance for a 8-disk SATA RAID0 array from 200 to 290 MBytes/s (45 % improvement). Using a ramdisk, peak performance improves from 320 to 474 MBytes/s (48 % improvement), which is 72 % of the maximum possible throughput in our experimental setup. We also analyze the remaining system bottlenecks, and find that although commodity storage systems have potential for building high-performance I/O subsystems, traditional network and I/O protocols are not fully capable of delivering this potential