Whats the max speed of the SD card slot ?

[obarthelemy]

Hi.

I don't remember reading anything about how fast the SD card slot on the RPi is. Performance seems to go up to 20MB/s for a class 10 card that's twice as expensive as a lowly class 4, and I'm sure we have even lowlier stuff gathering dust in our drawers. Is it worth spending the extra money ?

[Svartalf]

Most modern embedded devices can cope with a Class 6 card. Many will work fine with a Class 10.

It should be noted, however, that the speed you're talking about is the write speed- and that you can have cards as hot as 30MB/sec in the Class10 space. Read speeds can be higher than the write speeds or at least the stated 10MB/sec as given by their Class rating (What number they have is the minimum assured read/write speed out of the device, not peak...).

[Archangel5089]

Reminds me, Amazon are selling Sandisk Transcend 16GB SDHD 'Class 10' for £15.... It's a faulty batch with a crippled write speed and they're just trying to get rid of them, do not fall for it!

I would also quite like to know what class the hardware can cope with...

[Svartalf]

Quote from Archangel5089 on August 31, 2011, 15:23
Reminds me, Amazon are selling Sandisk Transcend 16GB SDHD 'Class 10' for £15.... It's a faulty batch with a crippled write speed and they're just trying to get rid of them, do not fall for it!

Is it a faulty batch (which would be ILLEGAL to sell) or a situation where people don't understand the Class definitions properly? Class 10 does NOT mean it will do 15, 20, 25, or even 30MB/sec read/write. All Class 10 means is that the device is claimed/certified to handle 10MB/sec sustained speed on read or write on a device capable of Class 10 operation. Nothing more, nothing less. Many of the vendor claims of "up to 30MB/sec" and similar are peak speeds on optimal devices that can cope with the data rates. All bets are off once you're outside the configuration they used to measure that "up to 30MB/sec" data rate. Some will do as good or better. Some will not do as well. If it meets the claimed Class 10 rating, regardless of what you might feel about the marketing, etc. it's not "faulty".


I would also quite like to know what class the hardware can cope with...


I'd venture at least 6.

[lingon]

Yes, I agree that it would be interesting to know if the SD-interface supports 4-bit transfers mode and what the maximum transfer rate is? According to Wikipedia you can have a transfer rate of 25 MB/s, 104 MB/s or a theoretical maximum of 312 MB/s.
http://en.wikipedia.org/wiki/S.....sfer_modes
I have a Transcend 8 GB SDHC 150x SLC SD-card with a Fedora 15 LXDE installation on it. The read speed on my card is about 18 MB/s and the write speed 12 MB/s after aligning the file system to the erase block size. SD-cards are small and simple devices so one cannot hope to achieve very high speeds as on real SSD-storage which has typically several flash chips read out simultaneously.

[Lob0426]

The simple answer is currently as high as 832Mbit/s(dependent upon standard). The more complex answer is we do not know what specification the SD card reader in RasPi has yet. The specifications determine which cards it can use. The different cards have different data transfer rates. Also the different cards have different PIN counts. They have as little as (nonstandard 4 pins) 7 pins to as many as 13 pins. My guess is it will fall within the MMC, RS-MMC, MMC Plus range of cards due to licensing. The most common cards have 9 pins on them which would limit you to MMC and RS-MMC (and SD licensed) which the standard is 20Mbits/s. The true SD standard, which requires licensing fees somewhere around $1000 to $4500 per year, is up to 832Mbit/s though the max SPI transfer is 50Mbit/s and 9 pins Which tops out at 416Mbit/s For MMC Plus. All of the SD style cards have royalties attached to them.

We need one of the Devs to weigh in on this for us.

[obarthelemy]

@Lob: Wow. That's informative, thanks. I had no clue the issue was that complicated... and started despairing at the first mention of a license ^^ Hopefully we'll know by the time the RPi comes out what's the best card we can stick in it without going overboard. It'd be nice to know indeed if there are non-obvious issues such a pin count, which I've never seen mentioned in any SD card description, only "class" and max throughput.

[liz]

We're still haggling over what SD card reader to use (every penny counts at this stage) - I may be able to let you know sometime next week if we've made a final decision.

[obarthelemy]

@ Liz:

[lingon]

Cost is obviously your primary concern for the Raspberry Pi, but the speed of the SD-reader is an important factor for the usability of the device. In interactive use the subjective feeling of the speed of a computer is largely determined by the storage system. Even a very slow computer can feel very snappy and fast running from a RAM disk or a fast SSD. Try to run your favorite Linux Live distro from a RAM disk to see what I mean. I would therefore suggest you to go for a SD-reader that can support speeds of 50 MB/s or 104 MB/s. The speed of the SD card is more important than the speed of the reader in practice, but it would be nice not to be limited to 25 MB/s, since the speed, size and cost of flash memory is developing fast at the moment.

[Lob0426]

Sorry obarthelemy, it really is not complicated for us, it is complicated for the foundation.
They have to decide which they are going to go with. The information is available in Wikipedia. I find the information fascinating. I never realized how complicated it might be to get a product off the ground. It seems like everything has a royalty attached to it. I would love to see a list of what components the foundation is going to pay either royalties or annual fees too. I would also like to know if any of them waived the fees or royalties for the foundation. It literally comes down to the bang for the buck for the foundation. And yes I am doing it again.

My guess is that we will be buying the good old run of the mill SD cards that are available everywhere. We just won't know what speed it can read at.

[lingon]

The maximum USB2 speed in practice is 25 - 30 MB/s because of the overhead in the protocol so it is tha same as the transfer speed 25 MB/s of standard SD-card readers. Having a faster SD-card reader in the Raspberry Pi with a speed of 50 MB/s or 104 MB/s would allow for a storage performance of about 2 or 4 times that of a standard SD-reader or USB2, which would be a significant performance gain.

[Svartalf]

Quote from Lob0426 on September 1, 2011, 00:05
Sorry obarthelemy, it really is not complicated for us, it is complicated for the foundation.

Welcome to the wonderful world of product development!


It seems like everything has a royalty attached to it. I would love to see a list of what components the foundation is going to pay either royalties or annual fees too. I would also like to know if any of them waived the fees or royalties for the foundation. It literally comes down to the bang for the buck for the foundation. And yes I am doing it again.


It either has a royalty or a production cost associated with it. When someone flings a "It only costs $x for this part, why don't you add WiFi (or feature "Y")?" they miss the reality that that's the parts cost and not the total cost of that device. It must be added in. It must be validated that it will work with the circuit as you've added. It must be added to the boards. It must pass regulatory compliance for emissions. Each one of those steps has a consequence and an associated expense. Once you've done all of this, adding WiFi, as an example, can add $15-25 to the end cost of the R-Pi.


My guess is that we will be buying the good old run of the mill SD cards that are available everywhere. We just won't know what speed it can read at.

Probably not until just short of when we'll be placing orders for them. Big burning question would be...why are we worrying about buying the "right cards" until the thing's about to ship?

[Lob0426]

Hint they asked us what 2GB card cost in our area. Almost all of these are 9 pin SD cards. In my area that is all that is available. 13 pin SDIO and MMC Plus is not available here at all. Pretty good idea thats what we will end up with, SD. I got a couple lying around. If not i will buy online.

[lingon]

Is there any news on what the maximum transfer speed the SD reader will be and what speed the ARM core interface itself supports?

[jamesh]

Quote from lingon on September 16, 2011, 15:54
Is there any news on what the maximum transfer speed the SD reader will be and what speed the ARM core interface itself supports?


See this thread (where you have already posted...)

http://www.raspberrypi.org/for.....ostid-7216

[lingon]

That other thread is about something slightly different than what I would like to have answered in this thread. A standard SD card can give a maximum read speed of less than 20 MB/s and then achieving a good write speed is about tuning for flash memory constraints. What I would like to know here is does the SD-reader and the ARM core support read speeds of 50 MB/s or even 104 MB/s? A standard magnetic hard disks gives you of the order of 100 MB/s read speed under optimized conditions with a large latency. There are already SD cards available with almost the read spead of magnetic hard disks and the huge advantage of low latency of course.
http://www.toshiba.com/taec/ne.....10_600.jsp
I have not yet found out what these costs, but the price of flash memory is dropping fast...
If the ARM SoC supports a fast SD reader on The Raspberry Pi, then we would have a nice hierarchy of I/O and memory bandwidth
Ethernet 12,5 MB/s
USB 2.0 25-35 MB/s
Fast SD-reader 50 or 104 MB/s ???
RAM ???
Cache ???

[lingon]

The pictures of the beta boards shows the RAM to be Hynix H9TKNNN2GDMPLR-NDM which is of type 800 MHz DDR2:

http://www.hynix.com/inc/pdfDo.....Memory.pdf

In theory the RAM bandwidth is 3,2 GB/s. The latency is also very important from the performance view, but I don't know what it is.

The Raspberry Pi SD-card reader is optimized for lowest cost, so I guess it is not of the highest speed, but it would be nice to know whether the BCM2835 supports UHS-I speeds or not?

[SwampPuppet]

lol @enthusiasts trying to pimp their raspberries.

The Pi is £15. You are all talking about read speeds of 20mb/s. That means the sd card is going to be more expensive than the raspberry pi. If you want high speeds then you get an expensive card with high volume - They don't make 2 Gb cards with high read speeds. So back to reality. A £5 card like the one below is 2Gb and reads at 5mb/s writes at 1.5mb/s.

http://www.dabs.com/products/k.....038;src=16

^ You may consider that luxury.

http://www.dabs.com/products/s.....html?src=3

^ You may consider this as alien technology.

[MaXMhZ]

Read speed of the SD card is not too important.

Latency is much more important.

I have booted Ubuntu 10.4 in under 3 seconds off an USB 2.0 (15MB/s) Micro SD (TransFlash) card.

[Vindicator]

If you watch the video's of a Raspi booting, they seem to take approximately 20 to 30 seconds on those video's.

I believe a class 4 card is probably good enough and they can be purchased for around 5 to 9 dollars in my local area.

The Raspi cost 25 to 35 dollars US and if it took 10 minutes to boot I would still buy it as the nearest competitor against a raspi cost about 100.00 dollars US.

It would be nice to have almost all of the suggested improvements but by the time it happens we would be paying 100.00 dollars or more. See Beagle board/Panda board Etc.

I don't think those boards will be cost effective for school children as you can buy refurbished PC desktops for that price.

okay mine was 9+12 and I ran out of fingers and toes.

[Jessie]

Right now, from what I've gathered, class 6 is as high as you can go, not due to the hardware but a Linux issue that at some point will be sorted out.  The class ratings mean nothing for the most part anyway.  A class 6 card is supposed to write at 6 MB/sec, and a class 10 at 10 MB/sec.  But I have seen plenty of benchmarks that show class 4 and 6 cards way out performing class 10 cards.  The read speeds are not defined by their class so gathering their raw reed speed by class is not going to happen.  Do google searches and you will find the true performers out there.

The other problem at hand is that for some reason many of the new class 10 cards are real flaky.

[lingon]

Great news to have the BCM2835 data sheet available! Thanks to all authors who have taken the time to put this nice book together!

On page 82 it says

UHSMODE

Select the speed mode of the SD card: RW 0x0
000 = SDR12
001 = SDR25
010 = SDR50
011 = SDR104
100 = DDR50
other = reserved

so the SD controller can sustain higher speeds than the USB-port. Now the questions is whether the present drivers support the capabilities of the hardware or not. With the documentation available UHS support can be implemented if it is not already done.

Obviously the write speed for many different streams is the most crucial thing for good flash storage, but good read performance (latency and bandwidth) is also important for the usability of the system. You can test this for your self by booting an old PC with a Live Linux CD to read the CD completely into a RAM disk. Applications will start very fast in this way from the RAM disk and the computer will be very responsive.

Software updates, disk checks during boot, file copying from the SD card to the USB-port and general usage will greatly benefit from good performance of the SD card. Fast SD cards can of course be used in other devices than the Raspberry Pi, so it makes sense to go for the best one can find at a reasonable price.

See this thread for how to optimize for the limitations of flash storage:

http://www.raspberrypi.org/for.....h-memory 

[INCSlayer]

lingon said: 


The pictures of the beta boards shows the RAM to be Hynix H9TKNNN2GDMPLR-NDM which is of type 800 MHz DDR2:

http://www.hynix.com/inc/pdfDo.....Memory.pdf

In theory the RAM bandwidth is 3,2 GB/s. The latency is also very important from the performance view, but I don"t know what it is.

The Raspberry Pi SD-card reader is optimized for lowest cost, so I guess it is not of the highest speed, but it would be nice to know whether the BCM2835 supports UHS-I speeds or not?


I know this is offtopic but if it uses a variant of DDR2 ram is it possible to down the line see a version of the r-pi with a SODIMM slot for expandable RAM?

[rpt]

INCSlayer said:


I know this is offtopic but if it uses a variant of DDR2 ram is it possible to down the line see a version of the r-pi with a SODIMM slot for expandable RAM?


No, as it uses PoP memory.