Does using both card slots slow down burst shooting and buffer clearing?

Question

If I set a dual-card camera (for example, a Canon 5D Mark IV) to write the same RAW files to both cards for backup, will that reduce performance? I'm mainly concerned about continuous RAW shooting and how quickly the buffer clears. If one card or slot is slower than the other, does the camera have to wait for both cards before freeing buffer space?

dual-card burst-shooting memory-cards canon-5d-mark-iv buffer

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

8y ago

user69369 · Accepted Answer

My simplified diagram of camera I/O:                                    |―――――――\
                                                                        ]        \
                                                                 /----> | CARD 1 |
|――――――――――|       |―――――――――――――――|       |――――――――――――――|      |      |        |
|          |       |               |       |              |      |      |________|
|  SENSOR  | <---> |  DATA BUFFER  | ----> |   CARD BUS   | ---->|
|  + DSP   |       |     (RAM)     |       |  CONTROLLER  |      |      |――――――――|
|__________|       |_______________|       |______________|      |      |        |
                                                                 \----> | CARD A |
                                                                        |        |
                                                                        |________|

Usually, the output buffer is the same for both slots, even if they support different media (e.g. Compact Flash vs. Secure Digital). That means that for 2 media that are of the exact same speed (and if both slots support that), there is almost¹ no performance difference between using 2 vs. only 1 card.

¹ _{There might be some difference if the card bus speed is too low - that might come down to parallelisation issues and/or the controller's speed.}

However, because of that I/O-design, the camera has to wait for the slowest medium to finish so it can clear the buffer - meaning that if your card A has a lower max. speed than your card 1 (or vice versa), the speed will fall back to that of the slowest medium, which means that performance will suffer.

For this, it is also necessary to remember that the maximum achievable speed is not necessarily the max. speed printed on the card:

Some manufacturers claim speeds that are not achievable outside a laboratory setting,
If the camera has a lower write speed than the card, then that will be the decisive number. Putting a 200 MB/s SD-card inside a 60 MB/s reader will lead to 60 MB/s speed at best.
Also note that when using UHS-II cards in UHS-I slots, it might actually make a higher-tier card slower than the lower-tier one. Also, compatibility might be an issue (in hopefully rare cases).

Practical, non-scientific examples from my EOS 5D Mark III:

How many consecutive, high-burst shots until the buffer is filled:

Using a UDMA 7 CF (SanDisk Extreme 32GB, 120MB/s max., video class 20) alone:
- Unlimited JPEGs / at least 14 RAWs
Using the same CF with a secondary SD-Card, Kingston, 4GB, Class 4:
- 1 RAW / 4 JPEGs
Using the same Class 4 SD-card alone:
- 1 RAW / 4 JPEGs
Using the same CF-card with a SanDisk Extreme PRO 32GB 95 MB/s max. UHS-I 3 Class 10 card:
- 12 RAWs / unlimited(?) JPEGs.
Using the same UHS-I 3 Class 10 card alone:
- 12 RAWs / unlimited(?) JPEGs.

Unfortunately, I do not have any other (slower) CF-card, so I cannot test it the other way round.

Note that the EOS 5D Mark III does not support UHS-protocols in SD-cards.

Limitations and thoughts:

All stated above only holds true for using the same quality settings for both cards. If you use a mixed setting, then performance could, in theory, be degraded since more DSP work is needed. Though as long as you write the larger file to the faster card, you should not notice this "DSP-lag".

Also, since camera manufacturers usually do not offer schematics of the process, all of my statements above should be seen as very simplified working theory. Not all manufacturers might do this in the same way.