Five rules to help you build a storage strategy


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In 1965, Gordon Moore of Intel Corp. noticed that the density of transistors on integrated circuits was doubling approximately every two years. In the 1980s, Robert Metcalfe of 3Com Corp. noted that the value of a network increases exponentially as more people use it.

Let's take a stab at defining a few axioms for storage.

1. Density increases, but unit price stays constant

Prices tend to remain constant from generation to generation, but functionality increases with time. The average cost of capacity drops as density increases. The price of a specific disk drive model or tape cartridge starts high, and then eventually drops. But the next generation comes in at the same high initial unit cost.

Enterprise disk drives usually start at about $1,300, and then drop below $400 as they are phased out. Tape media starts at $200 and falls to $50. Consumer disk drives usually start near $500 and fall off the shelves at $80 (before rebates).

2. Backup tapes are three times larger than disks

Enterprise tape and disk units maintain a rough ratio of capacity of approximately three or four to one. In 1994, the DLT2000 had 15GB (native) capacity, and most enterprise arrays were just adopting 4GB disks. Today's 160GB to 300GB tapes map to 36GB to 140GB enterprise disks.

Backup tapes have to be somewhat larger than disks to stay in the storage game. If they fell behind, disk-based backup would take over.

3. Storage density doubles

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every 18 months

This axiom sounds like Moore's Law, but not really. Most computer components benefit from Moore's exponential growth of transistors, but disk doesn't rely on transistor counts for increased density. Different science and engineering allows us to put more and more on a small magnetic platter.

Disk capacity had been growing a steady 25% annually. But the introduction of the magneto resistive (MR) disk head in 1993 changed everything. Suddenly, magnetic disks were matching--and sometimes exceeding--transistor density growth. Since then, storage density has grown 60% annually, giving us the same 18-month doubling cycle as the semiconductor industry.

Enterprise and consumer flush time

This was first published in June 2004

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