MicroSD card speed for 4K video

DMKAlex wrote:

sludge21017 wrote:

You need to pay more attention to speed ratings that are on the cards.

Samsung EVO select 128gb from a quick google shows me U3 rating on the card.

That means something. V30 is on other cards that are newer. That's another speed rating.

These are U3 and V30, and cheaper.

https://www.microcenter.com/product/626486/micro-center-premium-128gb-microsdxc-card-uhs-i-flash-memory-card-c10-u3-v30-a1-micro-sd-card-with-adapter

(I use the 512GB U3/V30 versions).

Interesting.

I try to stay with brand name cards like Sandisk or Samsung. I don't know MicroCenter has their private label.

You should be good to go with your Samsung EVO select 128gb card. It's U3 rated by Samsung, which implies it's good for a minimum of 30MByte/s sequential writes, which should be fine for video at 200Mbit/s, or a little more.

Best way to be sure is to try it in your camera at maximum bitrate.

The SD Association has a few comments here: https://www.sdcard.org/developers/sd-standard-overview/speed-class/

You asked what kind of transfer sizes are used in video recording on a camera. Generally as big as possible, for efficiency in both time and power. The figures for 64kByte-256kByte transfers are probably what you should be looking at, though there are likely to be some smaller transfers as well, partly depending on the container format (MOV,MP4,MKV,...), and on how the audio and video streams are prepared for writing within the camera.

As you can see from the figures in your test, there is a point of diminishing returns at large write sizes. At some point the camera wants to recover the memory holding the encoded frame data to be written, to store data for a new frame. It can do that sooner if it doesn't wait until it's got megabytes of data to write just so it can write enormous blocks. Available buffer memory can be huge - one RAW file's worth of buffer memory is likely to be good for a a second or more of compressed video.

The performance figures from your test aren't necessarily representative of sustained write performance on a card that isn't brand new, or for writing that fills up a significant proportion of ther card: The speed and durability of the memory in most cards is not uniform (for good reasons), and sometimes the card will need to pause internal writing to erase internal flash blocks, or re-arrange data across internal flash blocks. This may explain some of the gap between the 30MByte/s minimum sustained write speed quoted by Samsung, and the 84MByte/s write speeds you see in your tests.

The SD Association provides a low-level formatting tool intended to optimise formatting for the internal card organisation, and, if requested, erase the entire card (reducing delays due to block erasure on-the-fly):

• https://www.sdcard.org/press/thoughtleadership/the-sd-memory-card-formatterhow-this-handy-tool-solves-your-memory-card-formatting-needs/
• https://www.sdcard.org/downloads/formatter/

Sometimes playback can be more demanding than recording. It's possible to have a container format that's optimised for writing in large blocks, but requires lots of small reads on playback - say to locate and recover chunks of audio data.