Next: Design Up: Introduction Previous: Introduction

## Hierarchical Web versus File System Caches

Our 1993 study of Internet traffic showed that hierarchical caching of FTP files could eliminate half of all file transfers over the Internet's wide-area network links. [10]. In contrast, the hierarchical caching studies of Blaze and Alonso [2] and Muntz and Honeyman [17] showed that hierarchical caches can, at best, achieve 20%hit rates and cut file server workload in half. We believe the different conclusions reached by our study and these two file system studies lay in the workloads traced.

Our study traced wide-area FTP traffic from a switch near the NSFNET backbone. In contrast, Blaze and Alonso [2] and Muntz and Honeyman [17] traced LAN workstation file system traffic. While workstation file systems share a large, relatively static collection of files, such as gcc, the Internet exhibits a high degree of read-only sharing among a rapidly evolving set of popular objects. Because LAN utility files rarely change over a five day period, both [17] and [2] studies found little value of hierarchical caching over flat file caches at each workstation: After the first reference to a shared file, the file stayed in the local cache indefinitely and the upper-level caches saw low hit rates.

In contrast to workstation file systems, FTP, WWW, and Gopher facilitate read-only sharing of autonomously owned and rapidly evolving object spaces. Hence, we found that over half of NSFNET FTP traffic is due to sharing of read-only objects [10] and, since Internet topology tends to be organized hierarchically, that hierarchical caching can yield a 50%hit rate and reduce server load dramatically. Claffy and Braun reported similar statistics for WWW traffic [7], which has displaced FTP traffic as the largest contributor to Internet traffic. . .

Second, the cost of a cache miss is much lower for Internet information systems than it is for traditional caching applications. Since a page fault can take 105 times longer to service than hitting RAM, the RAM hit rate must be 99.99%to keep the average access speed at twice the cost of a RAM hit. In contrast, the typical miss-to-hit cost ratio for Internet information systems is 10:1 , and hence a 50%hit ratio will suffice to keep the average cost at twice the hit cost.

Finally, Internet object caching addresses more than latency reduction. As noted above and in the file system papers, hierarchical caching moves load from server hot spots. Not mentioned in the file system papers, many commercial sites proxy all access to the Web and FTP space through proxy caches, out of concern for Internet security. Many Internet sites are forced to use hierarchical object caches.

The Harvest cache has been in use for 1.5 years by a growing collection of about 100 sites across the Internet, as both a proxy-cache and as an httpd-accelerator. Our experiences during this time highlight several important issues. First, cache policy choices are made more difficult because of the prevalence of information systems that provide neither a standard means of setting object Time-To-Live (TTL) values, nor a standard for specifying objects as non-cacheable. For example, it is popular to create WWW pages that modify their content each time they are retrieved, by returning the date or access count. Such objects should not be cached. Second, because it is used in a wide-area network environment (in which link capacity and congestion vary greatly), cache topology is important. Third, because the cache is used in an administratively decentralized environment, security and privacy are important. Fourth, the widespread use of location-dependent names (in the form of Uniform Resource Locators, or URLs) makes it difficult to distinguish duplicated or aliased objects. Finally, the large number of implementations of both clients and servers leads to errors that worsen cache behavior.

We discuss these issues in more depth below.

Next: Design Up: Introduction Previous: Introduction