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If you have been following the storage business for a while, you will have noticed a few changes:

  • Introduction of Flash Memory components as Solid State Disks
  • Serial ATA (SATA) disks becoming popular and growing in capacity (2TB soon)

There are lots of other disk technologies around like Fibre Channel and SAS but SSD and SATA are getting the big press and are taking market share.  You might ask why? Disks in a data centre, use power day and night, 365 days a year. A typical disk (Seagate Cheetah 15K.4 147GB SCSI) uses about 18W. In a data centre that means that it’s lifetime (5 years) power consumption including cooling and power protection (PUE 1.6) is likely to be 1.26 MWh. At 10c per KWh that equates to $126 per disk. So for 1PB of storage the lifetime cost of power will be $860,000 not including capital plant.

So getting the power that disks use down to a reasonable level is important. The formula that engineers quote for power consumption is:

Power ∝ Diameter 4.6 x RPM 2.8

So if we use large physical disks like in the old days where 8″ and 14″ were common disk formats we get 7717 times more power needed to drive a 14″ disk than a smaller 2″ one.

Size

Power of 4.6

Ratio to 2″ disk

14

187149.63

7717.04

8

14263.10

588.13

3.5

318.21

13.12

2

24.25

1.00

So the world is moving to smaller and smaller disks to reduce power demand, reduce heat output and deliver increased densities.

Spin speed has a similar impact so low spin speed disks use a lot less power than their high speed equivalents.

Speed

Power of 2.8

Ratio to 5400 RPM

15000

493236861172.40

17.47

10000

158489319246.11

5.61

7200

63172925606.55

2.24

5400

28229457196.94

1.00

Slow spin speed, small disks use less power than larger high spin speed disks.

As the price of SSD continues to drop, the high spin speed disks that we use for high IOPS solutions will increasingly become replaced with SSD, whilst capacity will be served by low spin speed SATA migratting the storage world to Flash and Trash.

Inevitable and proved by the maths.

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  • Andy Sparkes

    Yes you are right, but its going to take awhile The cross over point is probably in the 2015-17 timeframe where SSD’s will become the defacto choice. Another interesting thing to consider is whats happening to I/O in different parts of the datapath. I think disk I/O is advancing at around 20% of capacity and if you examine whats happening to network I/O a similar picture of asymmetry appears. Therefore you are also seeing the whole memory hierachry is changing with SSD’s effectively extending the cache layer and then thngs can get really interesting.