Google's Data centers.
IP Addresses and Domains of Google's Data Centers
The progression of a Google Dance could basically be watched by querying the IP addresses of Google's data centers. But queries on the IP addresses are normally redirected to www.google.com . However, Google has domains, which resolve to the single data centers' IP addresses. These domains as well as their IP addresses are shown in the following list.
Domain IP Address
Note: Searches at www-zu and www-sj are currently redirected to other data centers. Since results for searches at their IP addresses fluctuate heavily during a Google Dance, also these searches seem to be internally routed to other data centers.
Those that keep an eye on Google's index updates often think that the Google Dance is over, when they see the new index at www.google.com or when they don't see the old index at www.google.com for some time. In fact, the update is not finished until all the domains listed above provide results from the new index.
The index updates at the single data centers seem to happen at one point in time. As soon as one data center shows results from the new index, it won't switch back to the old index. This happens most likely because the index is redundant at each data center and at first, only one part of the servers (eventually half of them) is updated. During this period, only the other half of the servers is active and provides search results. As soon as the update of the first half of servers is finished, they become active and provide search results while the other half receives the new index. Thus, from the user's perspective, the update of one data centers happens at one point in time.
www-ab.google.com, www-cw.google.com, www-dc.google.com, www-ex.google.com, www-fi.google.com, www-in.google.com, www-sj.google.com, www-va.google.com and www-zu.google.com point at Google's datacentres.
When those nine return the same backlinks, that would normally be the sign that the update has finished. It's best to think of www.google.com, www2.google.com, www3.google.com and toolbarqueries.google.com as load balancing domains that can be pointed at datacentres.
www2.google.com and www3.google.com have been pointed at datacentres with the new index some time before www.google.com starts to show it. There's no reason why any of these four domains shouldn't swap between old and new indices during the update process, hence the term 'Google dance'.
The unusual thing this time is that the link: search and Toolbar PageRank returned by the nine www- domains appear to be old. It would seem sensible to expect a second half to this update, unless Google have something else up their sleeve.