How to Choose a Hard Drive -

How to Choose a Hard Drive


How Real-World Numbers Make the Case for SSDs in the Data Center

This article on How to Choose a Hard drive was updated on Feb. 27, 2014.

Last month I reviewed a blog from Backblaze on how not to choose a hard drive. This month I will weigh in on how to choose a hard drive for consumers and enterprises. I selected the following vendors and drives:

Toshiba-Consumer, enterprise 3 TB, enterprise 2.5 inch 15K RPM and enterprise SSD

HGTS-Consumer, enterprise 4 TB, enterprise 2.5 inch 15K RPM and enterprise SSD

WD-Consumer, enterprise 4 TB, enterprise and 2.5 inch 15K RPM (No SSDs were listed)

Seagate-Consumer, enterprise 4 TB, enterprise 2.5 inch 15K RPM and enterprise SSD

I chose the latest drive for each vendor as of January 31 2014, and collected all of the information from the each of the vendor’s web sites. I spent a fair amount of time looking around each vendor’s the web sites as best as I could for the most detailed document available.

Documentation from Seagate was the best, followed very closely by HGST, with WD far behind and Toshiba even farther behind. In the first part of this article, I'll cover consumer and 4 TB enterprise drives and, later in this article, I'll look at 2.5 inch 15K RPM drives and SSDs. (And as a reminder, HGST has been purchased by WD.)

Consumer Drives The first thing you will notice with consumer hard drives is that for many vendors there is a lack of documentation details like MTBF (mean time before failure) or MTTF (mean time to failure). This is especially true when comparing consumer drives to enterprise drives.

what is private cloud, define private cloud

*Average rate of <55TB/year. The MTBF specification for the drive assumes the I/O workload does not exceed the average annualized workload rate limit of 55TB/year. Workloads exceeding the annualized rate may degrade the drive MTBF and impact product reliability. The average annualized workload rate limit is in units of TB per year, or TB per 8760 power-on hours. Workload rate limit = TB transferred × (8760/recorded power-on hours).

** Cannot determine as you have to put details about the customer or drive in to get details.

***MegaScale DC addresses low application workloads that operate within 180TB per year.

****Note WD40EFRX (AKA Caviar Red) lists 1M hours but this is a different drive.

So what does all of this information tell us about consumer hard drives? I find it very interesting that both HGST and Seagate list an average rate of I/O for a year. The Seagate number comes to .12% of the year at 104 hours running at full rate of the drive, while the HGST comes to 4.57% utilization of the year at 400 hours.

I am sure that both hard drives have counters which can tell you how much data has been transferred. Using these drives at home is likely fine unless you are doing lots of video editing or similar workflow, in which case it is likely that you will exceed these values. But I would find it pretty difficult for a home user to exceed these values. The only thing I could think of to challenge a home system is video surveillance.

I think there are a few other areas to consider.

1. The hard error rate for all of these hard drives is 1 sector in 10E14 bits read. This is another area that will not likely impact the home user but tells me that the drives are not designed for enterprise applications.

2. The drives are designed to operate between (55C) 131F and (60C) 140F at the upper range, which is again fine for the home users and is similar to what is found for enterprise drives.

3. There is not much information on seek latency for these hard drives, but there never has been much data. For the home user this does not matter much.

4. Warranties are much less than for enterprise drives for 3 of the 4 vendors, with Toshiba being the exception. I guess being third requires you to try harder.

The issues that I would consider for a home drive are: how will you be using it. Are you going to hit the write performance issues? That would be the big question that I would need to answer.

Clearly you are going to be outside the vendor specification for the HGST and Seagate drives using them in a commercial backup application. The performance range for the vendors for MB/sec is about the same except for Toshiba, which did not provide any data. For a home system – which is generally running a lower performance file system and a single application – the data rates listed are all reasonable. Those are the issues I would consider but I would never ever use these drives for a commercial application.

From what I can tell and remember the duty cycle documentation that Seagate and HGST have is reasonably new news. My guess is that these vendors are clearly stating that these drives should not be used in heavy I/O applications and if that is your workload you should be using enterprise drives.

Enterprise Nearline (AKA Business critical) These are the 4 TB drives that were SATA a few year ago and now have both a SAS and data interface. Some of the things to look at in these drives are different than consumer drives. Note for performance I am using the SAS interface values if provided and vendors often have multiple models for each of these drive families.

I am using the self-encrypting drives with secure erase if it is available as part of the table below. Since the encryption is done in hardware the vendors state it does not significantly impact the performance.

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Sources: HGST / Seagate / Toshiba / WDC

*HGST provided lots of details on ECC and error correction in Ultrastar_7K4000_SAS_Spec_V1.7.pdf, page 27

**Seagate provides a detailed discuss of reliability in chapter 5.0 RELIABILITY SPECIFICATIONS, which is the manual for this drive found on the website.

***States that the MTBF is based on a value of 40°C ****Seagate and Toshiba provided the rotational latency number, not the seek+latency.

*****Product MTBF and AFR specifications are based upon a 40°C base casting and system workloads of up to 180 TB/year (workload is defined as the amount of user data transferred to or from the hard drive).

******Toshiba states that they have “disk to disk data protection application.” This could be ANSI T10 PI/DIF but it is not clear.

Most of the drives in the category, for most users, will be going into RAID controllers and you will be getting these drives from your storage vendor. In most cases for large RAID vendors and even small RAID vendors they qualify multiple vendors disk drives.

I think the key consideration that people need to be concerned with for the duty cycle of the WD drive is: they are really not meant for the enterprise environments and therefore this drive should not be used for enterprise applications. Add to this the lack of SAS support for the WD drive.

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