OldMarty
Posts: 7
Joined: Wed Dec 19, 2012 12:34 pm

SD card speed - important???

Sun Dec 23, 2012 2:06 pm

Hi All,

Just wondering if a faster SDcard will make any NOTICABLE difference with overall performance of R-pi ????
I don't know how fast the R-pi can access the SDcard, and it might be pointless to buy an expensive high-speed SDcard if R-pi simply can't access it quickly enough...

When shopping around, the immediate options are:
SDcard - "Ultra" 30Mb/sec.
SDcard - "Extreme" 45Mb/sec.
SDcard - "Extreme Pro" 95Mb/sec.

I already have some "Extreme" (45Mb/sec), from my digi-cams, and tried 8gb, 16gb and 32gb all with great success using R-pi.
I'm wondering now if i go up a notch to "Extreme Pro" (95Mb/sec) if i'm going to see a great improvement or nothing at all?

Best Regards,
Marty.

User avatar
pluggy
Posts: 3635
Joined: Thu May 31, 2012 3:52 pm
Location: Barnoldswick, Lancashire,UK
Contact: Website

Re: SD card speed - important???

Sun Dec 23, 2012 4:32 pm

The speed rating of a card relates to write speed, its for sustained writes as when you're recording an HD video on an SD camcorder. The Pi will spend most (90-95%) of its time reading. They all read faster than they write and some slower ones will read faster than supposedly faster ones.

You'll hear something completely different from those who have spent a lot of money because they swallowed the hype. There is also a fair body of evidence that faster cards are more problematical on the Pi.

http://elinux.org/RPi_Performance#SD_card

You want a good read speed, its a WiKi so it isn't definitive.
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

efflandt
Posts: 359
Joined: Mon Dec 03, 2012 2:47 am
Location: Elgin, IL USA

Re: SD card speed - important???

Sun Dec 23, 2012 5:34 pm

I doubt it would be any faster. The fastest I have seen for hdparm buffered reads on a USB 2.0 hard drive is around 32 MB/sec vs. about 23 MB/sec for same drive on a powered hub on Raspberry Pi.

This is a 30 MB/sec class 10 SDHC on a Pi:

Code: Select all

[email protected] ~ $ sudo hdparm -tT /dev/mmcblk0p2

/dev/mmcblk0p2:
 Timing cached reads:   302 MB in  2.01 seconds = 150.33 MB/sec
 Timing buffered disk reads:  60 MB in  3.07 seconds =  19.52 MB/sec
This is a 35 MB class 10 SDHC on USB 2.0 in 8 GB 64-bit Ubuntu Linux desktop:

Code: Select all

[email protected]:~$ sudo hdparm -tT /dev/sdc2

/dev/sdc2:
 Timing cached reads:   9894 MB in  2.00 seconds = 4949.22 MB/sec
 Timing buffered disk reads:  56 MB in  3.08 seconds =  18.19 MB/sec


While hdparm is just a simple read comparison, not a complete read/write test, results for your 45 MB/sec card (if any better or not) might reveal if it is worth a more expensive card that may not be any faster on the Raspberry Pi, but can be larger than 32 GB if SDXC works for you.

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