It's Happening - μSDXC Cards At The Speed Of SSD's


Yes, at least because these are UHS2 cards with extra pins. The Pyra can only interface against standard UHS1 pins at most, so the cards must fall back to those speeds (which I don't have on me at present). And on top of that the wiring from the ports has various bandwidths - the three different SD slots run at different speeds IIRC.
 
The advertised speeds don't mean much (to us).

SD cards are optimized for sequential r/w, because that's what is needed for photo bursts and video recording.
What we want is random r/w, because that's what matters when accessing a lot of small files, like libraries and configs of a linux system. SD cards are much worse on this matter than SSDs, even if their sequential speeds look somewhat similar.

This site [1] has a nice amount of benchmarks of different SD cards, including random r/w. According to these tests Sandisk Extreme UHS I U3 SDXC cards seem to be the fastest for our use-case, both for full-size SD and µSD. Other Sandisk Extreme and Samsung's Pro Plus µSD cards come close.
What you can see here is that even the best cards don't reach 4MB/s in random r/w, while real SSDs are between 20 and 100MB/s afaik (my knowledge here might be outdated).
You can also see that having a UHS II card doesn't seem to help at all unless you have a UHS II card reader (which the Pyra doesn't).


[1] http://www.cameramemoryspeed.com/reviews/sd-cards/
 
Last edited by a moderator:
An excellent resource, thanks.

I'll just note that since this is the offtopic thread (not a pyra one), this may not be the absolute best advice for all who might come across this thread. But as a reply to a.ginger.turtwig (as my post as also) it's good advice.
 
The advertised speeds don't mean much (to us).

SD cards are optimized for sequential r/w, because that's what is needed for photo bursts and video recording.
What we want is random r/w, because that's what matters when accessing a lot of small files, like libraries and configs of a linux system. SD cards are much worse on this matter than SSDs, even if their sequential speeds look somewhat similar.

This site [1] has a nice amount of benchmarks of different SD cards, including random r/w. According to these tests Sandisk Extreme UHS I U3 SDXC cards seem to be the fastest for our use-case, both for full-size SD and µSD. Other Sandisk Extreme and Samsung's Pro Plus µSD cards come close.
What you can see here is that even the best cards don't reach 4MB/s in random r/w, while real SSDs are between 20 and 100MB/s afaik (my knowledge here might be outdated).
You can also see that having a UHS II card doesn't seem to help at all unless you have a UHS II card reader (which the Pyra doesn't).


[1] http://www.cameramemoryspeed.com/reviews/sd-cards/
This should probably be added to the wiki. Once I am caught up for the day I will try to remember to do it.
 
One more thing to consider when talking about SD card speeds:

SD cards come pre-formatted with a certain file system. For SD and SDHC this is FAT32, for SDXC it's EXFAT. In most cases people will keep this file system. However, we don't. To install Linux we need a file system that supports the UNIX file permission system, like one of the EXT family.
Most benchmarks you'll find will be based on the default file systems. That should give you a good idea about the general performance of the card, but these results might not be transferable to other file systems. Flash memory can be optimized for a certain type of file system. So if you use another file system, the performance of the card might be lower.
On rare occasions a card might even be completely useless when used with a non-default file system. I've had that with a USB stick, that works fine with NTFS, which it was shipped with, but it doesn't work reliably at all when formatted with any other file system.

So we might want to keep our own list of SD card benchmarks with cards formatted with a "Linux friendly" file system.
A useful benchmark for Linux,. that can provide results for random r/w seems to be "fio" [1], which is also available as a Debian package [2]. Note, that "gnome-disk-utility", although popular for being easy to use and providing a nice graphic output does not test random r/w, so it's not really useful here.


[1] http://freecode.com/projects/fio
[2] https://packages.debian.org/search?keywords=fio&searchon=names&exact=1&suite=all&section=all
 
Back
Top