What exactly is the Wait Time Rate for WRITELOG measuring?
This figure is derived from the sys.dm_os_wait_stats DMV. (http://msdn.microsoft.com/en-us/library/ms179984.aspx)
The wait time rate is calculated as the difference between successive executions of the DMV divided by the time elapsed between the two executions.
The column wait_time_ms is described as "Total wait time for this wait type in milliseconds. This time is inclusive of signal_wait_time_ms.". In other words, it includes the time between when the waiting thread was signalled and when it commenced running. So, this time includes far more than just the disk latency.
Another possible cause for the figure discrepancy would be if the SQL Server instance is run on a virtual machine. VMware may be making the CPU clock jump when it starts running a VM after it has been stopped - skewing the calculation because it is using a time difference.
