14

With the new Raspberry Pi 3 out, is the speed of the bus still limited to around 20 MB a second?

I am looking to purchase a micro SD card in it and I am not sure if I should spend the extra amount for a UHS-3 card that can get around 30+ MB/s speeds.

6

There have been no comments (at least from what I've seen) on the sd card slot of the Pi other than that it is a friction slot (like the Pi Zero), instead of a spring lock. It's safe to assume that the bus is still limited to 20Mb/s. I wouldn't worry about getting a UHS-3 card. The Pi 3's features are listed here.

You can see the spring lock here: (on the Pi 2)

enter image description here

And the friction slot here: (on the Pi 3)

enter image description here

  • would you recommend a uhs-1 or just a class 10? – Oscalation Mar 5 '16 at 21:19
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    @Oscalation According to this: sdcard.org/consumers/speed by the SD Card Association, UHS-1 and Class 10 have the same speed. Personally, I use Class 4 cards, because they are cheaper and write quickly enough for me. If I were you, I'd buy a Class 10 if you really need the speed because UHS-1 are more expensive. If you really need that full 20Mb/s, then go ahead and get a UHS-3 card, but you will be wasting the extra 10Mb/s that the card is capable of. Beware though, higher wright speeds usually means lower read speeds. – Patrick Cook Mar 5 '16 at 21:22
14

The speed of the microSD card I/O is exactly the same on the Pi 3 as it was on the Pi 2, and the major limitation (in terms of raw throughput) is the fact that the bus it's on is limited to ~20 MB/s by default, or ~40 MB/s if you have a UHS card and overclock the card reader.

Note that overclocking the card reader has a tendency to make less reliable microSD cards flake out, and can cause complete data loss/corruption, so if you overclock the reader, you do so at your own risk!

That said, I've tested a huge variety of microSD cards on the Pi 2 and Pi 3, at both normal clock rate and overclocked, and found that the current best cards (like the Samsung Pro+, the SanDisk Extreme and Extreme Pro, and the Samsung Evo+) can reach sequential read/write speeds of 40 MB/s and random write speeds of 2-4 MB/s.

In most Pi usage, the random read/write speeds will impact the experience more, and in that regard, even the fastest microSD can't touch the performance of the slowest disk drive or SSD you might use on a normal computer.

2

There is a lot of idle speculation about SD Card speed. All the standards for 6/10 etc cards guarantee is minimum write speed. This is mostly related to video recording or similar activities writing data in bursts.

Unless you are regularly saving large files there is no difference, and as Patrick Cook says higher write speeds may come at the expense of read, which is more significant for most Pi applications. I see no difference between 4/6/10 cards.

The standards do not specify read speeds which differ between card manufacturers.

All of this needs to be taken with a grain of salt. Most (if not all) manufacturers do not recommend using SD cards for OS.

0

You can benefit from hi-speed SD cards for off-line backup/install tasks, and reuse them for nextgen pi with faster IO

-1

Jeff Geerling posts some extensive tests on his blog: Raspberry Pi microSD card performance comparison - 2015, way too much information to adequately summarize here, but the Samsung EVO+ and Pro+ cards are leading the pack in terms of speed.

  • 2
    Welcome to Raspberry Pi! Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. – Steve Robillard Mar 16 '16 at 3:26
  • This is a link only answer. Links, almost inevitably, tend to die over time, so when that happens your answer will not be particularly helpful. If you can provide a synopsis of the links content, that would make the link much more useful. – Greenonline Mar 16 '16 at 4:23
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    I can tell you, as the author of said post, that I have kept links active for over 15 years now, making sure to always add redirects if the content changes. That said, very few web authors are as dedicated to not breaking the web as I am, so I'll post another relevant answer with more info here :) – geerlingguy Jun 12 '16 at 3:14

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