I think I know what you mean, assuming by disk cache you mean something like a write cache on a hard disk or SSD. If so, read on. Checkpoint only watches for an acknowledgment from Windows passed to it from the I/O subsystem. Thus, data is changed in the data cache. The checkpoint tells SQL Server to harden that page(s) and SQL Server passes those pages to Windows to the I/O subsystem which sends back an acknowledgment that the data has been hardened. However, neither Windows nor SQL Server knows if that data was written to a cache on the SAN or to the lowest level (disks/SSDs) of the SAN. That's the nature of the beast.