Methods and apparatus relating to processing out of order transactions for mirrored subsystems. A first device (that is mirroring data from a second device) includes a cache to track out of order write operations prior to writing data from the write operations to memory. A register may be used to track the state of the cache in response to receipt of a special transaction, which may be a posted transaction or snapshot. The first devise transmits an acknowledgement of commitment of the data to memory once all cache entries, as recorded at a select point by the register, are emptied or otherwise invalidated. Devices may communicate via a peripheral component interconnect express (PCIe) interconnect, and may include a point-to-point or serial link. Various components may be on the same integrated circuit die. An uninterrupted power supply or batteries may supply power in response to a power failure.