As a growing number of web sites introduce mirrors to increase
throughput, the challenge for clients becomes determining which mirror
will offer the best performance when a document is to be retrieved.
In this paper we present findings from measuring 9 clients scattered
throughout the United States retrieving over 490,000 documents from 47
production web servers which mirror three different web sites.  We
have several interesting findings that may aid in the design of
protocols for choosing among mirror servers.  Though server
performance varies widely, we have observed that a server's
performance relative to other servers is more stable and is
independent of time scale.  In addition, a change in an individual
server's transfer time is not a strong indicator that its performance
relative to other servers has changed.  Finally, we have found that
clients wishing to achieve near-optimal performance may only need to
consider a small number of servers rather than all mirrors of a
particular site.