Before Data ONTAP saves data to the disk, it collects written data in NVRAM. Then, at a point in time called a consistency point, it sends the data to disk.
As data is added or changed, the data is not directly written to the disk. Changes or additions to data are temporarily stored in the NVRAM. Then, at a consistency point, SnapMirror writes the data to the disks on the destination system. When the synchronous SnapMirror feature is enabled, the source system forwards data to the destination system as it is written in NVRAM. Then, at the consistency point, the source system sends its data to disk and tells the destination system to also send its data to disk. Finally, the source system waits for the destination system to acknowledge that it sent data to disk before continuing with the next write.