12.17 Flash Storage

An interesting storage medium that has recently become popular because of its compact form factor ^[3] is flash storage media. The flash medium is actually a semiconductor device, based on the electrically erasable programmable read-only memory (EEPROM) technology, which, despite its name , is both readable and writable. Unlike regular semiconductor memory, flash storage is non-volatile , meaning that it maintains its data even in the absence of power. Like other semiconductor technologies, flash storage is purely electronic and doesn't require any motors or other electro-mechanical devices for proper operation. Therefore, flash storage devices are more reliable and shock resistant, and they use far less power than mechanical storage solutions such as disk drives . This makes flash storage solutions especially valuable in portable battery- powered devices like PDAs, electronic cameras , MP3 playback devices, and portable recorders .

Flash storage modules now provide between 2 MB and several gigabytes of storage, and their optimal price point is at about $0.25 per megabyte. This makes them far more expensive per bit than hard disk storage, which means that their use as a storage medium is diminished.

Flash devices are sold in many different form factors. OEMs (original equipment manufacturers) can buy flash storage devices that look like other semiconductor chips and then mount these devices directly on their circuit boards . However, most flash memory devices sold today are built into one of several standard forms including PC Cards, CompactFlash cards, smart memory modules, memory sticks, or USB/flash modules. For example, a digital camera user might remove a CompactFlash card from their camera, insert it into a special CompactFlash card reader on their PC, and access their photographs just as they would files on a disk drive.

Memory in a flash storage module is organized in blocks of bytes, not unlike sectors on a hard disk. Unlike regular semiconductor memory, or RAM, you cannot write individual bytes in a flash storage module. Although you can generally read an individual byte from a flash storage device, to write to a particular byte you must first erase the entire block on which it resides. The block size varies by device, but most OSes will treat these flash blocks like a disk sector for the purposes of reading and writing. Although the basic flash storage device itself could connect directly to the CPU's memory bus, most common flash storage packages (such as Compact Flash cards and Memory Sticks) contain electronics that simulate a hard disk interface, and you access the flash device just as you would a hard disk drive.

One interesting aspect to flash memory devices, and EEPROM devices in general, is that they have a limited write lifetime. That is, you may only write to a particular memory cell in a flash memory module a certain number of times before that cell begins to have problems retaining the information. In early EEPROM/flash devices, this was a big concern because the average number of write cycles before failures would begin occurring was around 10,000. That is, if some software wrote to the same memory block 10,000 times in a row, the EEPROM/flash device would probably develop a bad memory cell in that block, effectively rendering the entire chip useless. On the other hand, if the software wrote just once to 10,000 separate blocks, the device could still take 9,999 additional writes to each memory cell. Therefore, the OSes of these early devices would try to spread out write operations across the entire device to minimize damage. Although modern flash devices still exhibit this problem, technological advances have reduced it almost to the point where we can ignore it. A modern flash memory cell supports an average of about a million write cycles before it will go bad. Furthermore, today's OSes simply mark bad flash blocks, the same way they mark bad sectors on a disk, and will skip a block once they determine that it has gone bad.

Being electronic, flash devices do not exhibit rotational latency times at all, and they don't exhibit much in the way of seek times. There is a tiny amount of time needed to write an address to a flash memory module, but it is nothing compared to the head seek times on a hard disk. Despite this, flash memory is generally nowhere near as fast as typical RAM. Reading data from a flash device itself usually takes microseconds (rather than nanoseconds), and the interface between the flash memory device and the system may require additional time to set up a data transfer. Worse still, it is common to interface a flash storage module to a PC using a USB flash reader device, and this often reduces the average read time per byte to hundreds of microseconds.

Write performance is even worse. To write a block of data to flash, you must write the data, read it back, compare it to the original data, and rewrite it if they don't match. Writing a block of data to flash can take several tens or even hundreds of milliseconds .

As a result, flash memory modules are generally quite a bit slower than high-performance hard disk subsystems. Technological advances are improving their performance, a process that is mainly being driven by high-end digital camera users who want to be able to snap as many pictures as possible in a short time. Though flash memory performance will probably not catch up with hard disk performance any time soon, it should steadily improve as time passes .

^[3] In this context, 'form factor' means shape and size .