FlashPage: A read cache for low-latency SSDs in web proxy servers

Abstract

The paper introduces FlashPage, a high-speed SSD caching system designed for ultra-fast media, with the goal of enhancing web page delivery in proxy servers. Traditional SSD caching schemes, designed primarily for slow HDD-based primary storage, encounter difficulties when applied to capacity-class SSDs as primary storage. This limits the high-performance capabilities of caching media. To address this issue, FlashPage operates within the Linux virtual filesystem layer, shortening the hit-handling path and minimizing lookup overhead. It incorporates a compact radix tree to fast locate cached data. These approaches reduce the software overhead for a 4kB read hit by over 5 times. FlashPage also employs novel admission and eviction policies to minimize flash wear while maintaining a high hit rate. As a second-level storage cache, FlashPage predicts the hotness of potential demotion candidates in the first-level storage cache (i.e., page cache), achieving a 10.1% higher hit rate and reducing write traffic by 10.4% compared to LRU. Evaluations using Varnish and Squid HTTP caches show its effectiveness, with up to 29.6% and 38.2% faster web request processing compared to Bcache and DM-Cache, state-of-the-art caching schemes in mainline Linux kernels.

Keywords

• SSD cache
• Tiered storage
• Web cache