Is Mirroring on one card is done through camera or through cards?

Question

It was a vague title, let me explain:) Some cameras for example Nikon D-610 and D-810 that I have used have two slots for memory cards and in settings you can tell it to use Slot2 as a mirror for the card in slot1 , so whatever shot you take on Slot1, it also saves it on card in slot2. One thing came to my mind was that what is the "Source" for Slot2? Does it copy from Slot1 to Slot2 or camera writes at the same time to both slots? Reason for asking about this is that I want to see if it is a good idea to do mirroring option as a backup solution in case files on card1 in slot1 get corrupted, I will still have correct file on card2 in slot2 but if the corrupted file from slot1 is getting overwritten to slot2 then it is garbage in, garbage out.

Answer 1

Yes, it is a good idea to use the configuration you describe for mirroring/backup.

The general algorithm is (unfortunately) in many models single-threaded in its storage part (IOW from step3 onward) and is as follows:

1. The picture is shot and is placed in camera's internal buffer in a queue structure.
2. The storage's subsystem I/O listener checks if the buffer's queue is (not) empty and if it isn't, it sets a picture pointer P0 to the first item (picture) from there
3. The picture referenced by P0 is saved to Slot1 with the format specified
4. When the step 3 finishes successfully - check the settings for backup/mirroring. If the appropriate setting is "ON", then save on Slot2 the picture referenced by P0 with the appropriate format
5. If the step 4 finishes successfully then mark the memory space referenced by P0 (our picture) as free. The camera 'sees' that the picture was 'deleted' from buffer.

The I/O (storage) layer is single-threaded - or, using your words, "the camera does NOT write at the same time on cards but sequentially" because it is easier to implement it like this (no need for inter-thread communications, listeners, more complex/expensive hardware etc.)

Of course, there are cameras which have multi-threaded I/O - or, using your words "the camera writes at the same time on both cards" - like my Canon 5D3 and, I presume, other top-of-the line Canon models (1DX, 7D2 etc.). Dunno about Nikon. For a multi-threaded approach the steps 3 and 4 above are executed simultaneously while in step 5 we have a listening thread which waits for 3 and 4 to signal their success in order to mark the space referenced by P0 as free and signal 'Ok, Terminated.' to the camera's main processing loop.

In any case, the second slot is written from buffer and not read from S1 not only because of readability/reliability factors as you noted but also because of speed considerations: camera's internal buffers are in fact very very quick cache memory chips which are many orders of magnitude faster than any CF or SD storage chain (controller+card) available.

Answer 2

I'm guessing, I don't know the internals, but stop and think about it...

The camera clearly already has the file buffered in its memory, from which it writes the first card. Why would it would it take the time to read it back (to write the second card), when it already has it buffered in memory? It is possibly multi-threaded, and writes both at the same time? Things like focus or metering and continuous shutter are surely multi-threaded.

EDIT: I just checked my D800, and the write time is about twice as long with two cards. But not three times as long.