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This article covers the use of solid state disk devices in FreeBSD to create embedded systems.
Embedded systems have the advantage of increased stability due to the lack of integral moving parts (hard drives). Account must be taken, however, for the generally low disk space available in the system and the durability of the storage medium.
Specific topics to be covered include the types and attributes of solid state media suitable for disk use in FreeBSD, kernel options that are of interest in such an environment, the rc.initdiskless mechanisms that automate the initialization of such systems and the need for read-only filesystems, and building filesystems from scratch. The article will conclude with some general strategies for small and read-only FreeBSD environments.
The scope of this article will be limited to solid state disk devices made from flash memory. Flash memory is a solid state memory (no moving parts) that is non-volatile (the memory maintains data even after all power sources have been disconnected). Flash memory can withstand tremendous physical shock and is reasonably fast (the flash memory solutions covered in this article are slightly slower than a EIDE hard disk for write operations, and much faster for read operations). One very important aspect of flash memory, the ramifications of which will be discussed later in this article, is that each sector has a limited rewrite capacity. You can only write, erase, and write again to a sector of flash memory a certain number of times before the sector becomes permanently unusable. Although many flash memory products automatically map bad blocks, and although some even distribute write operations evenly throughout the unit, the fact remains that there exists a limit to the amount of writing that can be done to the device. Competitive units have between 1,000,000 and 10,000,000 writes per sector in their specification. This figure varies due to the temperature of the environment.
Specifically, we will be discussing ATA compatible compact-flash units, which are quite popular as storage media for digital cameras. Of particular interest is the fact that they pin out directly to the IDE bus and are compatible with the ATA command set. Therefore, with a very simple and low-cost adaptor, these devices can be attached directly to an IDE bus in a computer. Once implemented in this manner, operating systems such as FreeBSD see the device as a normal hard disk (albeit small).
Other solid state disk solutions do exist, but their expense, obscurity, and relative unease of use places them beyond the scope of this article.
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