Memory Cards -- Is Speed Important?

From Internet Tablet Talk

The short answer for many users is 'No, the speed of a memory card is not important'.

In the case of the N800 (and maybe the N810) the tablet itself is not be capable of writing to a memory card much faster than about 2-3 MB/sec. A class 2 memory card, or a 40X SD card is sufficiently fast for the N800.

For the 770, Nokia actually throttles the I/O speed so that it will work with slow cards. If you want to take advantage of a fast card then you must configure your tablet to boot from a custom kernel installed on the memory card instead of booting from the internal flash memory in the tablet - see below.

Some longer answers, does a high speed card make any difference:

Q. On a stock machine doing web browsing, or using regular apps?

A. No. (However, one user reports a significant difference in Canola scanning between a class 6 and a class 2 card that he owns).

Q. When showing video or playing audio?

A. No.

Q. When copying files from a host PC over the USB connection?

A. Yes, but within limits.

Writing to a really slow memory card takes longer than to a fast card, but not by an enormous factor - maybe 50% longer. The general consensus from this discussion: http://www.internettablettalk.com/forums/showthread.php?t=5118 is that the N800 seems to cap at about 2MB/sec write speed. If you plan to copy files directly to your tablet then you will not see much benefit from using SD cards that are faster than class 2.

Q. When copying files directly to the card inserted in a USB reader?

A. Yes.

This is the big win for high-speed cards - but only if you copy big files (e.g. movies) or lots of files (e.g. audio files) to your card.

This assumes that your PC and USB memory card reader both support high-speed transfer speeds using USB 2.0 - not USB 1.0 or USB 1.1. Most PCs and memory card readers built since about 2002 support USB 2.0.

Q. When booting the tablet from a high speed card?

For both 770 and N8x0: Yes, as long as you use a custom kernel that enables fast memory card I/O. This is available from fanoush (see below).

Memory Card Speeds

There are two ways that memory card speeds are denoted.

SD cards are denoted by a transfer speed similar to the way that CD transfer speeds are quoted: as a multiple of 150kB/s (almost certainly read speed rather than the much slower write speed), so:

  6x  0.9 MB/s
 40x  6.0 MB/s
 66x 10.0 MB/s
133x 20.0 MB/s 

If the card does not show a speed rating on its label, then it could be almost anything up to a 66x card as many off brand names are simply re-packaged branded cards.

Note that these speed ratings say nothing about how fast you can write to an SD card, but it is safe to expect that writing will be slower (maybe much slower) than reading. You probably want to try to find a value for the write speed for any card that you plan to use in your tablet.

SDHC cards are rated for the speed of writing where the speed is the minimum sustained write speed to empty SDHC cards:

Class 2: 2 MB/s
Class 4: 4 MB/s
Class 6: 6 MB/s

Note that these ratings say nothing about how fast you can read from a card, but it is safe to expect to be able to read faster (maybe much faster) than you can write to an SDHC card. Also, a Class 6 may be much faster than 6 MB/s, as there is no definition for class 8 or higher.

Booting from a memory card

This may boost performance as well as provide other benefits: more space to install applications and the possibility to back up your tablet's software to your PC.

This thread provides a 'howto' on booting from a memory card: http://www.internettablettalk.com/forums/showthread.php?t=8631

If you want to experiment with installing support for higher speed SD/MMC cards in a 770 (not for the N8xx tablets) then you will also need a kernel from here:

http://fanoush.wz.cz/maemo/#sdhc

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Note: I have tried to assemble this answer from a question I originally posted here: http://www.internettablettalk.com/forums/showthread.php?t=12771 and from scanning many posts elsewhere. I do not feel that I have really found a definitive answer, but this is the best I could come up with. I would really appreciate it if others more knowledgeable than I would correct any errors.