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Power Consumption

The testing methodology is explained in general in a separate article. I’ll give some more details about it here.

So, we test the power consumption of each hard disk drive in six modes:

  • Start-up
  • Idle
  • Random Read
  • Random Write
  • Sequential Read
  • Sequential Write

The Start-up mode is meant to show you the peak power consumption of the HDD at the moment its motor is started up (as you know, the start-up current of an electromotor is significantly higher than its operating current). An important note: this test, as opposed to the others, is performed over a “cold” HDD. That is, the HDD is kept at room temperature for about 10 minutes and then connected to the testbed. This imitates the typical scenario, a cold start of the PC.

The other tests are performed over a “warmed-up” HDD. I keep the HDD running in idle mode for 10 minutes (the HDD is powered up, its platters rotating, but there are no disk accesses).

This help measure the power consumption more accurately because it depends on the temperature of the mechanical parts of the drive (the viscosity of oil in the motor and the friction force of the bearings of the heads block shaft) and the windings of the electromagnets.

The Idle mode, as explained above, imitates the situation when the PC does not access the hard disk for a while. The latter is free to spend this time in any manner. The HDD can scan its surface for potentially bad sectors, or rotate platters doing nothing, or even slip into sleep mode. The HDD has different kinds of sleep, by the way. It can keep the platters rotating but park the heads, or reduce the spindle rotation speed, or halt the spindle altogether. Of course, the deeper the sleep, the more time it takes the HDD to wake up from it and get ready for work.

The Random Read mode imitates a heavy load on the HDD. Each new read request has a new LBA address and the drive’s actuator is constantly on the move. Coupled with the typical operation mode of the motor, the load nears the maximum.

The Random Write mode is the same as the previous mode but the HDD is performing requests to write random-address data blocks. Using this load we can evaluate the effect of the deferred writing algorithms on the power consumption of a HDD. Theoretically, deferred write algorithms reorder write requests to optimize performance. Upon receiving a write request, the HDD decides if it is handy to perform the request immediately or later. As the result of the reordering, the HDD’s heads move along an optimal route instead of running wildly throughout the entire platter. Of course, the HDD spends less power when performing reordered requests.

The Sequential Read mode is designed to show the power consumption of a HDD when the latter is processing sequentially placed data (e.g. reading large files or playing video files with a high bit rate). Practice suggests that the load on the HDD electronics is quite high in this mode.

The last test mode is called Sequential Write. Like with Random Write, we’ll see if the power consumption at sequential writing and reading is any different.

So, let’s start with the Start-up mode.

These are the peak currents on the +5V and +12V power lines. These numbers cannot be used to calculate the peak power consumption of the HDDs because the maximums for each voltage do not occur at the same moment. However, these data are enough to compare the HDDs.

You can see that the HDDs from Seagate and Western Digital require quite a high current on the +12V line while the Hitachi drives consume more from the +5V line. The results are very interesting.

Theoretically, it is the Hitachi HDDs that should consume more from the +12V line because they have one platter more than the HDDs from WD and Seagate. On the other hand, the peak +12V current depends on the time the HDD takes to speed up the platters. The 1-terabyte Deskstar is declared to have a typical Power-on to Ready time of 20 seconds whereas the Caviar GP takes only 13 seconds to accelerate its platters. Of course, I should also note that the WD drive has a different spindle rotation speed as you’ll learn shortly.

The Samsung is the leader in terms of power consumption at start-up in both currents. Well, it is logical that a HDD with fewer platters consumes less power. But that’s only the start-up time. Let’s see what we have in other operation modes.

 
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