DOM

8/14/2004

Disk On Module (DOM). DOMs are flash memory units that emulate an IDE disk They are mounted directly on a 40 or 44 pin IDE connector that will plug directly into an IDE cable. All logic and signaling of an IDE drive are provided by the DOM. The unit is powered through the IDE cable.

Similar units designed to plug into a bus are called Disk On Chip (DOC) DOCs They are similar to DOMs in most respects, but will generally require a driver in the OS whereas DOMs should use the existing IDE drivers. For Linux, the DOC driver is called an MTD device.

As of mid 2004, DOM units are selling for as low as 50 cents US per megabyte with prices dropping rapidly. Speed is comparable to a hard drive -- 30MBytes/second in PIO mode. Power consumption in low, less than a watt. Since there are no moving parts, DOMs are quiet compared to a disk. Capacities as of mid 2004 are 4MB to 512MB.

Because their cost is much higher than that of a hard drive, it is anticipated that DOMs will find use in applications where their advantages -- low heat creation, immunity to vibration, small size, quietness -- justify the high cost.

In addition to cost, DOMs are rumored to occasionally fail catastrophically if power is lost during a write. Reformatting the unit may bring it back into service -- or may not. Also, the number of writes to a bit in todays flash memory based DOMs may be as low as ten thousand. Thus, DOMs will perform best in applications where writes are rare. Swap space for example probably should not normally be allocated on a DOM In order to minimize write failures, some DOMs incorporate a "wear leveling" algorithm that ensures that successive writes of the same externally specified "sectors" are written to different areas of the flash memory. There are questions -- maybe specious -- about whether (all?) DOMs support PIO mode transfers properly.

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