SSDs and Fast Erase
The SSD Guy has run across some confusion lately about fast erase on SSDs. It’s time to clear this up.
SSDs can undergo a very fast erase, and have had that capability for a number of years. After ruggedness this is probably the key reason the military is so enamored with SSDs.
Let’s say that you were out to capture a major terrorism kingpin and your helicopter crashed. How would you assure that the information on the drives in that helicopter could not be recovered by the terrorist organization? Before the era of Self-Encrypted Drives (SEDs – the subject of an upcoming post) HDDs had a problem – the drive could only be erased through its single head one track at a time. This would take a number of hours, and power might be lost before the process was finished.
Engineers considered various solutions: Applying a strong external magnetic field to the HDD, or crushing it with hydraulics would consume a lot of energy, which might not be available, and the apparatus would add considerable weight to an aircraft. Explosives were considered, but since the drive is often in the cockpit console, above the pilot’s lap, this proved to be an unpopular idea.
SSDs have an advantage in this regard. An SSD is made of a number of individual flash chips (the orange boxes in the graphic), each of which can erase one entire block in tens of milliseconds, under the command of the controller (brown box.) Since such a command takes less than a microsecond to issue, overlapping block erases can be issued to every chip in the SSD, with those block erases starting within microseconds of each other. Once the first block is erased, then the next, and the next can be told to erase, until the entire array of all the chips has been converted to all ones.
All of these erases overlap, so the time it takes to erase the entire SSD is very little longer than the time it takes to erase a single chip. To erase 4,096 blocks in an SSD with 32, or 64, or 128 chips would take on the order of 2 minutes. NAND chips with half as many blocks would take half as long.
For some extremely sensitive applications a simple erase is not enough. Some SSD vendors support more elaborate erase modes that, after erasing, write random numbers into each bit to remove any possible ghost images of information that might remain. Once the entire drive has been filled with random bits, it is again erased, and the cycle repeats in a never-ending loop. If power is removed during the process it will begin where it left off as soon as power is restored. It is impossible to make the SSD exit this process once it has begun.