CSACs seem unnecessary. Within a few racks, PTP (IEEE 1588) can do a pretty decent job of getting things synchronized to tens of microseconds. Requires that y

CSACs seem unnecessary. Within a few racks, PTP (IEEE 1588) can do a pretty decent job of getting things synchronized to tens of microseconds. Requires that your ToRs and NICs support it, but that's not a very onerous requirement, particularly if you're GoogleBookSoft. This means you only need a few time receivers for a datacenter, along with careful monitoring and implementation of your time distribution, but that happens through your Ethernet switches. More: http://www.ni.com/newsletter/50130/en/ http://www.ni.com/newsletter/50130/en/ But you need your datacenters to stay synced even if you lose GPS. You can use an atomic clock for this (Cs or Rb). But, for the rest of us, a good GPS-disciplined double-oven crystal oscillator (XO) can get you within the range needed for Spanner, IIRC spanner's time sync requirements. For example, this little one: https://www.microsemi.com/document-portal/doc_download/133412-gps-2500-and-gps-2550 https://www.microsemi.com/document-portal/doc_download/13341... will do +- 7 microseconds / 24h holdover. ("holdover" == operating when it's lost GPS).