I wanted to find the fastest SD card for my Raspberry Pi, or at least one that wasn’t so slow that it was going to cause a bottleneck. There are a few threads on the raspberrypi.org forums about performance benchmarks. The prevailing opinion on the “SD Card Benchmarks” thread at raspberrypi.org is that the SanDisk Ultra SDHC 8 GB Class 6 cards have perhaps the best random read/write speeds, which people feel is a good, if theoretical, metric for how the Raspberry Pi will access the card. But at least one Raspberry Pi owner who had done some of the benchmarking that suggested SanDisk Ultra cards would be ideal discovered that the board would not boot from the card.

And there are enough other reports of SanDisk Ultra Class 6 cards not working to convince me to fall back to a regular SDHC Class 4 card.

With so much uncertainty around which cards will work, I’m keeping an eye on the growing list of known-good and known-bad SD cards for the Raspberry Pi at elinux.org.

Of the Class 4 SanDisk Ultra cards, SanDisk Ultra 4GB SDHC Class 4 Flash Memory Card SDSDH-004G-U46 is reported to work, but it requires that the power be disconnected and the card allowed to sit for a minute or two before it will reboot. That may be acceptable for a use case where the Raspberry Pi is sitting on my desk, but not one where it will be tucked behind other equipment.

I happened to be near a Walmart over the weekend, so I stopped in and picked up a SanDisk 4GB SDHC Class 4 Flash Memory Card SDSDB-4096-AW11. I benchmarked it with CrystalDiskMark, and found it to underperform my target of random write speeds of 1 MB/s.

-----------------------------------------------------------------------
CrystalDiskMark 3.0.1 x64 (C) 2007-2010 hiyohiyo
                           Crystal Dew World : http://crystalmark.info/
-----------------------------------------------------------------------
* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

           Sequential Read :    12.281 MB/s
          Sequential Write :    11.721 MB/s
         Random Read 512KB :    12.548 MB/s
        Random Write 512KB :     2.475 MB/s
    Random Read 4KB (QD=1) :     4.713 MB/s [  1150.7 IOPS]
   Random Write 4KB (QD=1) :     0.024 MB/s [     5.9 IOPS]
   Random Read 4KB (QD=32) :     4.977 MB/s [  1215.0 IOPS]
  Random Write 4KB (QD=32) :     0.025 MB/s [     6.2 IOPS]

  Test : 50 MB [H: 0.0% (0.0/3773.5 MB)] (x5)
  Date : 2012/06/01 20:34:28
    OS : Windows 7  SP1 [6.1 Build 7601] (x64)

Assuming it boots and reboots normally, I’ll probably use it until the community settles on a real-world benchmark that run on the Raspberry Pi.

And just for posterity, the proper way to format an SD card and ensure optimal performance is to use the SD Formatter software from the SD Association.

My Raspberry Pi was shipped today by element14, but I started researching power supplies last week. A page on www.soronlin.org.uk titled Raspberry Pi Power Supplies indicates that any cell phone charger capable of providing 0.7A (or 700mA) of current should suffice, but the author recommends choosing a name-brand unit, and preferably one that provides 1A (1000mA). A post on www.raspberrypi.org titled “The Boreatton Scouts meet a Raspberry Pi” describes a setup using an “old BlackBerry 700mA charger”, which the Raspberry Pi was fine with until a few USB peripherals were added.

I tend to over-provision, so based on the recommendation to use a 2 amp adapter in this forum thread at www.raspberrypi.org titled “Adequate Power Supply Critical to Pi Stability”, I picked up an HP TouchPad Power Charger for $4.99 with free shipping from the HP Home & Home Office Store. (That’s an awesome price, by the way.)

The HP TouchPad Power Charger

It’s reportedly the same charger that originally shipped with the TouchPad, and it’s a 2.0 amp charger, so it should charge just about anything with a USB port, short of a notebook PC. It comes with a one-year warranty from HP, too. A good number of the reviews on Amazon.com complain that the included USB cable is lousy, but I have a number of short, 24AWG USB cables with ferrite cores from monoprice.com laying around, so no worries there.

The specs

Specs taken from the side of the PSU:

Input: 100-240V~50-60Hz 0.4A
Output: 5.3V-2.0A

Specs taken from the page at HP:

Maximum Output Power: 10 W

My only concern is that the 5.3 Volts exceeds the 5.0 Volts that the Raspberry Pi is designed for, but I’m reasonably assured this won’t be a problem. I have a multimeter, so I’ll be testing the voltage across the test points.

Alternatives

There is a good list of verified peripherals, including a number of known-good power adapters, at the RPi VerifiedPeripherals page at elinux.org. The HP Touchpad supply is listed among the known-good adapters, but it’s referred to as a “5V 2A Charger for HP Touchpad”.

Of course, monoprice has some USB chargers, too, and I’d probably pick up a 2.1A, 4 Port USB Wall Charger as a second choice (assuming it’s not still on backorder).