SD Reliability - Best Practices

Posted by: Thom Denholm

In an earlier blog post titled Managed NAND Performance: It's All About Use Case, we referred to an article measuring SD media, specifically sequential write performance. Speed was measured using a camera to shoot continuous pictures. Photographers and other users of SD media have another concern - reliability.

A short survey of photographer websites finds that physical damage causes the most problems with SD cards. This isn't so much because the internal NAND flash has been damaged, but as a result of the access pins and internal connections failing. This is less of a concern for devices that use SD media as their internal storage, such as medical or industrial equipment. SD makes a lot of sense in those cases, as it can be replaced or upgraded.

Newer SD cards use newer NAND flash, and there are reports of problems with data retention. A good wear leveling solution in the firmware can help with this, as the blocks are worn evenly and rewritten more frequently. Firmware is not common between vendors, so buying a name brand can result in name brand firmware under the hood.

Another suggestion given on the photographer websites is to format the media instead of erasing files. This is most likely done to improve performance for large sequential file allocations. Another benefit is that the format only rewrites the FAT and root directory on the card, where individual file deletes will affect those locations and any subdirectory metadata. On an SD card, a format performs less writes than a series of deletes.

While cameras will likely continue to use the FAT file system for exchangeability, other devices using SD as storage media are not limited to this file system. Datalight's Reliance Nitro is an excellent fit for SD storage where data is critical and product life is long. An installable Windows driver is available if the SD card needs to be removed and accessed via a Windows-based desktop.

The one thing most SD manufacturers emphasize is that their cards will last for a long time. The average camera application writes large files, often sequentially, and these stay in place until downloaded and then deleted. This results in a very low count of writes and erases to the NAND media. With proper physical care, an SD card used in this fashion can last ten years or more. It is likely to be replaced for capacity reasons before then - the average size of an SD card in 2003 was 256 MB.

While the camera use case is particularly friendly to SD media, one that isn't is database applications - which are becoming more prevalent in multi-function devices like handheld terminals and smartphones. These applications generate many small writes which can greatly reduce the life of the flash media. For optimum SD card life these small writes should be grouped into fewer, larger writes. Of course, for reliability reasons, maintaining the order of the writes is critical. Datalight has addressed this need with our newest product, FlashFXe, which enhances Reliance Nitro to bring faster writes, power-efficiency and improved flash endurance while keeping the best reliability for the application.

Read more about FlashFXe


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