![]() ![]() The class rating of the card is not sufficient to know exactly how the card will behave under any other workload than the one that was tested! The cynical side of me thinks that controllers may be optimized to pass these workloads better just to qualify for higher class numbers. Arguably, in these situations, a card with the best combination of sequential, random, large and small block access speeds would be the best choice. When using SD cards for purposes other than with digital cameras, small block accesses can become dominant and the performance of the cards in this regard can vary widely. A very common use of cards other than digital cameras is for expansion of storage on laptops, tablets and smartphones which may involve running applications from the storage, or the use of embedded system platforms running Linux on the SD card itself, such as the Raspberry Pi, Beaglebone Black etc. Likewise, readers and other devices may not clearly state whether they support certain cards and transfer modes, and become the “bottleneck” of an otherwise fast card. ![]() Despite cards being advertised as “compliant” to the SD specification, you may occasionally find a card which behaves “slowly” or incorrectly with certain devices, leading to data corruption or loss. SDSC, SDHC, SDXC and controller “violations” of the specifications). This results in a minefield of possible issues, one of which is compatibility (e.g. Each card’s performance is a function of the type of flash used (SLC, MLC, TLC with varying page sizes and interfaces which affects program times and error rates), the number of channels, and controller (wear-levelling, block management, ECC). Unfortunately, this doesn’t give us the complete picture. Cards with higher speed grades or UHS-I designation are likely to perform better than those with lower designations in general. Faster is Better?įor the most part, this allowed consumers to get a better idea of how the cards would perform for most of their use cases – e.g. These collectively were referred as the UHS modes, with UHS-I being now widely available. Lately, the original SD transfer mode of 25Mhz “double data rate” clocking with 4-lanes reaching a theoretical 25MB/s became a significant bottleneck, which led to the development of higher-clocked modes. These included Class 2, Class 4, Class 6 and Class 10 for a guaranteed speed of 2Mb/s, 4Mb/s, 6Mb/s and 10Mb/s respectively. These would be based upon testing to the SD Association’s rules. To address the growing need for reliable, faster storage with the advent of High Definition video and high resolution RAW photo shooting, the SD Association introduced the Class-rating system which defined a set of class levels for a guaranteed level of write performance under fragmented conditions. Unfortunately this made for confusion and disappointment as many of those ratings were made using specific conditions set by the manufacturer which were not always clear, and sometimes the ratings referred to read as opposed to write speeds (especially for lower end, low cost cards) as read speeds are easily achieved in comparison to write speeds. Others placed “vague” rates such as “Up to 20Mb/s” on their labels, none of which are clear as to whether this referred to read or write rates. Other manufacturers placed speed ratings, denoted by an “X”, where 1x referred to the CD-ROM transfer rate of 150kB/s. Ultra, Ultra II, Extreme III, Extreme IV), which were sometimes confusing. ![]() Early-on, flash manufacturers attached different monikers to their memory to confer their relative speeds (e.g. Most users are aware that flash memory comes in a variety of speeds and capacities. ![]()
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