Content-Based Block Caching

In this paper we propose a novel cache management mechanism termed the Content-Based Buffer Cache. The Content-Based Buffer Cache (CBBC) attempts to maintain a single copy of any block in memory according to its contents. In the presence of repeated content, this mechanism increases the effective size of the buffer cache. Overheads for maintaining this extra state information are small and bounded, providing an overall system performance improvement. Additionally, we eliminate writes to blocks where the new and old content are the same, reducing pressure on the I/O subsystems in the presence of these “Silent Writes”. We have logged traces of block-level disk access for a group of workstations over a several month period using a modified Linux kernel designed to boot off of an iSCSI target. We have analyzed single client access, as well as multiple client access to distinct logical disks using a unified block cache. There is significant replication of content and significant numbers of “Silent Writes” within a single workstation trace, improving the Content-Based Buffer Cache read hit rate as much as 80% over the traditional buffer cache design. We have also found that there is significant sharing of content between disks, which benefits content-based caching performance in the presence of a unified cache. For our workloads, these results indicate that content-based buffer caches dramatically improve I/O performance when used to manage a cluster of similar storage.