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Most people are familiar with the Russian Matryoshka nesting dolls. As you open each doll, it reveals another smaller one inside. Storage utilization has a parallel to these dolls: Each component in the chain of storage has its own method of capturing metrics and making improvements.

If you focus only on host utilization, you could miss wasted space on the array. Furthermore, you can compound waste by inflating projections for growth or buffer space. Put all of these wasteful practices together, and you can wind up with less than 20% of actual utilization, and think you're doing fine.

Measuring utilization should be simple: Divide the amount of storage used by the amount available for use, and you have your utilization percentage. But obtaining these metrics can prove tricky, and the frame of reference is key. Additionally, many systems have added overhead between the raw storage they can see and the usable storage they allow to be used.

For the purposes of measurement of storage utilization, we'll focus on the following three metrics:

  • Raw storage is the amount that's visible to a system before RAID protection, formatting and other overhead.
  • Usable storage is the amount that could hold user data.
  • Used storage is the amount actually taken up by this data.

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Buying storage in advance is wasteful
YEAR ITEM PURCHASE TODAY PURCHASE ANNUALLY SLOW GROWTH SLIM BUFFER
  Annual Growth 20% 20% 20GB 20GB
Buffer 50% 50% 50% 20%
1 GB Needed 120 120 120 120
GB Bought 310 180 180 180
Cost/GB $100 $100 $100 $100
Total Paid $31,000 $18,000 $18,000 $18,000
2 GB Needed 144 144 140 140
GB Bought 0 36 30 0
Cost/GB $50 $50 $50 $50
Total Paid $0 $1,800 $1,500 $0
3 GB Needed 172 172 160 160
GB Bought 0 42 30 12
Cost/GB $25 $25 $25 $25
Total Paid $0 $1,050 $750 $300
4 GB Needed 207 207 180 180
GB Bought 0 52 30 24
Cost/GB $12.50 $12.50 $12.50 $12.50
Total Paid $0 $656 $375 $300
Total GB 310 310 270 216
Total Cost $31,000 $21,506 $20,625 $18,600
Cost Savings 0% 31% 33% 40%

This was first published in February 2003

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