Highly Reliable Systems: Removable Disk Backup & Recovery


3.5 Inch Hard Drives Use Less Power than 2.5 for Backup

By Darren McBride

We spend a lot of time studying backup to removable disk.  Would it surprise you to learn that 3.5 inch hard drives use less power than 2.5 for backup on a per Gigabyte stored basis?

3.5″ disks have around twice the surface area of 2.5″ disks.  They also typically have 1.5 to 2 times as many platters.  No surprise that total storage per drive runs about 3-4 times larger for 3.5″ compared to 2.5″.  For the past 2 years (2010-2012) the largest available 2.5″ drive has been 1TB.  The largest 3.5″ drives have been 3TB, recently going to 4TB.

Let’s compare the power usage of a Seagate Barracuda 3TB 3.5″ drive running at 7200 RPM to a 1TB Seagate Constellation (ST9100640NS) running at 7200 RPM.  For this comparison we assume neither drive spins down or goes to standby mode:

1TB 2.5″ drive –  Idle – 3.31 Watts, Typical operating 5.21 Watts, PowerChoice mode 1.2 W
3TB 3.5″ drive –  Idle – 5.4 Watts, Typical operating 8.0 Watts, Sleep Mode .75 Watts

Let’s assume our backup job takes 8 hours per night and we need to backup 3TB:

Using 2.5″ drives – 3 drives *(5.21W*8Hrs + 3.31W*16Hrs) = 283.9 Watt*Hours per day
Using 3.5″ drives – 1 drives *(8W    *8Hrs + 5.4W*16Hrs) = 150.4  Watt*Hours per day

Conclusion: Backing up to a 3.5″ hard drive uses half the power
(assuming the drives don’t go into standby mode).

Obviously there are a lot of other scenarios we can imagine.  If both drives were in use 24 hours:
2.5″ drives – 3 drives *(5.21W*24Hrs) = 375.1 Watt*Hours per day
3.5″ drive1 – 1 drive * (8W*24) =              192 Watt*Hours per day
Still about a 2 to 1 power savings in favor of 3.5″

What if the drives spin down (sleep) when not in use?
Using 2.5″ drives – 3 drives *(5.21W*8Hrs + 1.2W*16Hrs) = 182.6 Watt*Hours per day
Using 3.5″ drives – 1 drives *(8W    *8Hrs + .75W*16Hrs) = 76  Watt*Hours per day
Now it’s 2.4 to 1 power savings in favor of 3.5″

What if the 2.5″ drives spin down but the 3.5″ drives don’t?
Using 2.5″ drives – 3 drives *(5.21W*8Hrs + 1.2W*16Hrs) = 182.6 Watt*Hours per day
Using 3.5″ drives – 1 drives *(8W    *8Hrs + 5.4W*16Hrs) = 150.4  Watt*Hours per day
So even if the 2.5″ drives spin down, we use less power with the 3.5″ drive.

 

 

 

Darren McBride

About Darren McBride

CEO, Highly Reliable Systems, Inc. View all posts by Darren McBride →


One Response to 3.5 Inch Hard Drives Use Less Power than 2.5 for Backup

  1. Andy says:

    The power consumption only works in favor of 3.5″ if you consider the power used per GB. But let’s say you only need 1TB, and have to choose between 2.5″ & or 3.5″, then it makes sense to go for 2.5″ drives isn’t it?

    Also considering your example, during backup, unless there’s a specific need to use all three 2.5″ drives everyday, then you save power by keeping the other two powered off and using only one. This reflects real world scenario more realistically as most people fill up one drive, then move on to another, they don’t keep some bits and pieces of data in each drive as backup.

    On the other hand, 3.5″ drives make sense for NAS because you will keep all the hard drives powered on 24×7 and at the same time you want the largest capacity possible to give you best bang for buck. So basically the use case scenario is important when talking about power savings.

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