Tag Archives: MTBF

Don’t just look at disk reliability!

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I am sure that many of you in the storage networking industry can relate to this very well.

When 1 or 2 disk drives fail, the customer will usually press you for an answer and usually this question will pop up. “How come the MTBF is 1.5 million hours but the drive(s) failed after a few months? We also get asked of “How reliable are the disks?” “How sure are you that the storage disks I buy will last?”

And for us in this line, we cannot deny the fact that the customer should be better informed (or at least we get cheesed off by these questions). A few blogs ago, I took the easy way out and educated the customer about MTBF (Mean Time Between Failure). This is only a quarter of the story because MTBF alone does not determine the reliability of the storage ecosystem and the reliability of the storage ecosystem (which translates to data availability) is something that the customer should ask rather than spending their time pressing their annoyance onto you about 1 or 2  disk failures.

I also want to say a little about another disk reliability statistics called AFR. More about that later.

Let’s get a little deeper with disk MTBF. Disk MTBF is a statistically calculated, pre-production measurement. The key word here is “PRE” meaning that THIS IS NOT A FIELD TESTED statistics! This is a statistical likelihood of how long a disk device will last.

One thing to note is how MTBF is derived. In fact, MTBF is established before the entire disk drive line goes into volume production. Typically, there is a process called Real Demonstration Test (RDT). RDT involves putting about 1,000 or more drives into a testing chamber, running them very hard, in elevated temperatures with 100% I/O for about 6-8 weeks. This is to simulate the harshest of an operating environment and inevitably, some disk drives will fail. From these failures, the MTBF is calculated.

A enterprise hard disk drives MTBF will usually be between 1.2 million to 2.0 million hours while the consumer grade drives usually have MTBF of about 300,000-600,000 hours. Therefore, it is important to educate customers because customers like to use some home office/SMB storage solutions to compare with the enterprise storage solution you are about to propose to him.

One of the war stories I heard was from a high-definition video production house. They get hundreds of thousands of Malaysian Ringgit worth of contract from a satellite TV content provider. But being less “educated” (could also be translated to being cheapo), they decided to store their valuable video contents on Buffalo NAS storage. And video production environments can be harsh. The I/O stress on the disks are strenuous and the Buffalo NAS disks crashed. They lost all contents (I don’t know what happened to their backup), and they were fined hundreds of thousands of Malaysian Ringgit and had their contract terminated on the spot. This is not to say that the Buffalo NAS is a poor product, but they got the wrong product for the job. You can’t expect to race the Formula 1 with an old jalopy, can you? You got to get the right solution for the job, even if it costs more.

So the moral of the story is – “Educate yourself and be prepared to invest if the dollar value of the data is more important than what are you think you might be cost-saving”

Over the years, MTBF (even though it is still very much in use today) is getting less and less useful as a reliability measurement. So, what’s better? AFR!

AFR or Annualized Failure Rate has been in use for almost 10 years now, and Seagate, the hard disk manufacturer, uses the AFR value heavily. AFR is the percentage of the installed bases of hard disk drives that failed and returned to factory in a given year. This is a more realistic figure and it is the statistics from the field. The typical value for enterprise disk drives  is usually between 0.7-1.0% although a few years ago, Google created a splash in the industry when they reported in an AFR of 36%. For those who would like to read Google’s paper, click here.

Therefore AFR is a more reliable measurement of disk reliability than MTBF.

But disk reliability is just a 1/4 of the story. We need to be out there educating the customers about the storage ecosystem reliability rather than a specific component. The data availability is paramount because components will fail throughout the lifecycle of the solution. That is why there are technology like RAID, snapshots, backup, mirroring and so on to ensure that the data is made available for the operations and businesses to continue.

Ultimately, if the customer wants to use the disk MTBF onto you, he’s basically shooting at you with the wrong bullet. It’s time you storage networking professional out there educate the customers.

Using simple MTBF to determine reliability to Finance

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The other day, a prospect was requesting quotations after quotations from a friend of mine to make so-called “apple-to-apple” comparison with another storage vendor. But it was difficult to have that sort of comparisons because one guy would propose SAS, and the other SATA and so on. I was roped in by my friend to help. So in the end I asked this prospect, which 3 of these criteria matters to him most – Performance, Capacity or Reliability.

He gave me an answer and the reliability criteria was leading his requirement. Then he asked me if I could help determine in a “quick-and-dirty manner” by using MTBF (Mean Time Between Failure) of the disks to convince his finance about the question of reliability.

Well, most HDD vendors published their MTBF as a measuring stick to determine the reliability of their manufactured disks. MTBF is by no means accurate but it is useful to define HDD reliability in a crude manner. If you have seen the components that goes into a HDD, you would be amazed that the HDD components go through a tremendously stressed environment. The Read/Write head operating at a flight height (head gap)  between the platters thinner than a human hair and the servo-controlled technology maintains the constant, never-lagging 7200/10,000/15,000 RPM days-after-days, months-after-months, years-after-years. And it yet, we seem to take the HDD for granted, rarely thinking how much technology goes into it on a nanoscale. That’s technology at its best – bringing something so complex to make it so simple for all of us.

I found that the Seagate Constellation.2 Enterprise-class 3TB 7200 RPM disk MTBF is 1.2 million hours while the Seagate Cheetah 600GB 10,000 RPM disk MTBF is 1.5 million hours. So, the Cheetah is about 30% more reliable than the Constellation.2, right?

Wrong! There are other factors involved. In order to achieve 3TB usable, a RAID 1 (average write performance, very good read performance) would require 2 units of 3TB 7200 RPM disks. On the other hand, using a 10, 000 RPM disks, with the largest shipping capacity of 600GB, you would need 10 units of such HDDs. RAID-DP (this is NetApp by the way) would give average write performance (better than RAID 1 in some cases) and very good read performance (for sequential access).

So, I broke down the above 2 examples to this prospect (to achieve 3TB usable)

  1. Seagate Constellation.2 3TB 7200 RPM HDD MTBF is 1.2 million hours x 2 units
  2. Seagate Cheetah 600GB 10,000 RPM HDD MTBF is 1.5 million hours x 10 units

By using a simple calculation of

    RF (Reliability Factor) = MTBF/#HDDs

the prospect will be able to determine which of the 2 HDD types above could be more reliable.

In case #1, RF is 600,000 hours and in case #2, the RF is 125,000 hours. Suddenly you can see that the Constellation.2 HDDs which has a lower MTBF has a higher RF compared to the Cheetah HDDs. Quick and simple, isn’t it?

Note that I did not use the SAS versus SATA technology into the mixture because they don’t matter. SAS and SATA are merely data channels that drives data in and out of the spinning HDDs. So, folks, don’t be fooled that a SAS drive is more reliable than a SATA drive. Sometimes, they are just the same old spinning HDDs. In fact, the mentioned Seagate Constellation.2 HDD (3TB, 7200 RPM) has both SAS and SATA interface.

Of course, this is just one factor in the whole Reliability universe. Other factors such as RAID-level, checksum, CRC, single or dual-controller also determines the reliability of the entire storage array.

In conclusion, we all know that the MTBF alone does not determine the reliability of the solution the prospect is about to purchase. But this is one way you can use to help the finance people to get the idea of reliability.