Thursday, September 1, 2016

Everything you Need to Know about Hard Drive Vibration


Hard Disk Drives (HDD's) are one of the most impressive and important electromechanical devices ever created. When I think about it, it is really amazing that these things actually work, let alone work so well!  These disks must quickly and precisely position heads slightly above very narrow tracks on rapidly spinning platters. If there is an error in the angular position of the head, it will not be positioned above the correct sector and therefore cannot correctly read or write data. 

One thing that can interfere with this amazingly fine and fast positioning is vibration. Vibration can cause small variations in the position of the head with respect to the track, so it can be an enemy of the proper functioning of HDD's.  But the mechanics of HDD vibration is an obscure subject, and as a result, there is an aura of mystery surrounding vibration. As part of my job, I've spent a lot of time and effort to understand vibration and to engineer our systems to deal with it.  I'm hoping to share some of that with you to help demystify the subject.


HDD Structure and Scale

The figure below shows the structure of a typical HDD, showing platters, tracks, heads, sectors, and cylinders (sets of corresponding tracks on platter surfaces).


To get a sense of how small all of this really is, let’s do some back of a napkin calculations.
  • A WD Purple 8TB HDD has approximately 248 million tracks cut up into just north of 15 billion sectors across all platters.
  • Let’s assume there are 8 platters in there and all are double-sided. That means there are ~15.5 million tracks on each surface of a platter.
  • Assuming 3.5” outer diameter and 1.75” inside diameter, the actual usable space of the platter has a radius of 0.875”.
  • That leaves us with 17.7 Million tracks per inch.
  • Considering the thickness of a piece of standard letter paper is 0.004” that means you could fit 70,800 tracks on the edge of a piece of paper.
That really is some tiny dimensions we're working with here.

Once you see how tiny the positional tolerances are, it becomes obvious that even the slightest vibration could move the arm (with respect to the platter) while reading or writing, resulting in an IO miss. As a result of this miss, the HDD must let the disk spin one revolution and try again. This greatly increases the time to complete the read/write, under extreme vibration, the operation may be delayed for several disk revolutions. 

These read/write delays are the root of all vibration-induced performance degradation.

Vibration Sources

I'm going to ignore vibration from sources outside the computer since it's more an issue for laptops and desktops, I work with servers which are mounted in racks and generally not subject to external vibration.

That leaves us with two main two main sources of internal vibrations: fans, and the HDD's themselves.

Fan Vibration - Fans are a source of vibration but good quality balanced fans have less vibration and a large heavy solidly built server, such as our 45 Drives' Storinator, will reduce fan vibration. If the manufacturer pays attention, fan vibration can be well under the threshold that HDD's can handle.  

Now, let's talk about vibration caused by the HDD's themselves. It comes two sources:  platter wobble, and reaction to head seek acceleration.

Platter wobble - Caused by a slight imbalance in the rotating platters. It is a readily identifiable sinusoidal vibration at a frequency that exactly corresponds to the rotational speed of the platter (90 Hz for 5400 RPM, 120 Hz for 7200 RPM, etc.). The dominant imbalance is in-plane so vibration is in the X-Y plane (see diagram below).

Reaction to head seeks acceleration HDD heads are mounted on arms, which rotate to seek to new tracks. These seeks can happen very rapidly, upwards of 100 seeks/second, so the arm must accelerate quickly.  According to the laws of physics, a rapid acceleration requires a large force, and each action has an equal and opposite reaction. Thus a rotational force is applied to the HDD body. Although the HDD body is massive relative to the head and arm, the reaction force is sufficient to slightly move the HDD, causing vibration in the X-Y plane.





A Direct Look at HDD Vibration  

The theoretical explanations are just fine, but it is much more interesting to actually see it. So we bought an accelerometer chip, wired its 3 axis outputs through op-amps. These were fed into a storage scope so we could record waveforms. Then I attached the acceleration to a Western Digital Ae 6TB HDD which I picked because I happened to have it on hand, and happens to have typical vibration characteristics.   I then recorded HDD vibration under two conditions: free floating (actually suspended on elastics); and attached to a massive block of granite. These show the maximum and minimum magnitudes of internal HDD vibration (free-floating shows worst case, HDD mounted to granite shows the best case, whereas HDD's mounted in a real server are an intermediate case).



Free Floating HDD - Maximum Vibration

The next two graphs were captured while the HDD was free floating. This represented the worse case drive mounting scenario. The Y Axis is the acceleration in G's (1G =9.8m/s^2), and the x-axis is time.

The first graph the vibrations are measured along the X-Axis. The Blue trace is when the disk is powered on but sitting idle. You can see that there is a consistent sine wave with a frequency of 90Hz, This is platter wobble. Converting that to RPMs and you get 5400 RPM, exactly what the disk is rated to spin at. The green trace was taken while the disk is under a random write test. You can clearly see peak accelerations that result from the head seeking the next sector. 


Soft Limit - The yellow lines are a soft limit around ~0.22 Grms where performance degradation will start to become annoying (exceeding %10). The vibration level causes position error that is great enough to cause an IO miss but not big enough to cause a full on timeout between the disk and the disk controller.

Hard Limit - The red lines are a  hard limit at ~0.67Grms, where the HDD will experience non-recoverable errors and the HDD will not operate in a useful fashion at all.







The next graph is the same setup, but displaying Y-Axis data (recorded at a separate time).  The blue trace is under idle conditions and the green trace is a during a random write.  Again, yellow is the soft limit, red the hard limit.





I haven't shown the Z-axis because vibration is minimal.





HDD's Rigidly Mounted to Granite 

The next graph was captured while the HDD was mounted to a large granite slab, this represented a best case drive mounting scenario. The blue trace is the vibration in the X-Axis and the Green is the vibration in the Y-Axis. Yellow is the soft limit.  (I did not bother to include traces during seek as the pattern is exactly the same as in the free floating scenario.)  Note that scale is different.  The dramatic reduction in vibration is obvious.




Observations and Conclusions

Note that in the free floating case, vibration is mostly under the soft limit, but on head seek the Y-axis vibration peak is just at the soft limit. The limit is an RMS number, so therefore we can conclude that if all hard drives had to worry about was their own excitation, vibration-induced degradation is not a problem.

If we rigidly couple the HDD to anything massive, vibration is dramatically reduced. Therefore a single HDD mounted to a chassis with no other sources of vibration will perform well,without any vibration problems.

Unfortunately, in the real world, a server typically has multiple disks, a large PSU with fans, and multiple cases and CPU fans. These all create a vibration which superimposes.  Thus one must carefully design the chassis such that HDD's are coupled to it with sufficient rigidity; and that the chassis itself has sufficient mass and rigidity in order to suppress vibration.  All this must be measured carefully. I'll be writing another blog entry showing what this all looks like.

Free feel to leave a comment below or contact us at info@45drives.com





          

Monday, May 9, 2016

How to Tune a NAS for Direct-from-Server Editing of 5K Video

At 45 Drives, we make really large capacity storage servers. When we first started out, our machines were relatively slow, and focused on cold-to-lukewarm storage applications; but our users pushed us to achieve more performance and reliability. This drove the evolution of our direct-wired architecture, which delivers enterprise level speed (read/write in excess of 3 Gigabytes/second) and reliability, without losing the simplicity and price point. 

All this extra speed made our Storinator servers highly suitable for a number of new applications, one of which is video editing. This field has seen its storage needs explode in recent years, and hugely benefit from our incredible capacity and density. Organizations have used them to centralize their storage, but the speed has opened up a whole new opportunity, namely to edit directly from the central server, rather than download and work off of internal hard drives. This saves time formerly required for transferring files to and from the workstation, but also can take place at a performance level higher than can be achieved from a single local drive (mechanical or SSD).

Thursday, April 7, 2016

Redundant FreeNAS Boot Drives on the Storinator!


At 45 Drives, the FreeNAS network attached storage operating system is a popular choice to run our high performance massive storage pods. It combines the advantages of the modern ZFS copy on write file system, with the simplicity of a purpose-built GUI driven NAS appliance shell.

Until today, whenever a customer received a Storinator running FreeNAS it would be configured with a bootable single USB stick that held the FreeNAS OS, and two SSDs configured as a backup of your FreeNAS configuration. In the unlikely event that the boot stick died, you simply had to plug in a new one (which could be created using the instructions and image on our wiki), and copy the configuration, and you're up and going again.

But a number of customers requested that we ship our FreeNAS Storinator pods with a mirrored install of FreeNAS directly on the SSDs, and use the USB stick as the NAS configuration backup. I have to admit, it really did make a lot of sense to me, so we did it!

Essentially we are flipping the old setup upside down. Instead of a single install backing up to mirrored devices we have a mirrored install backing up to a single USB.

ADVANTAGES

We feel that this new set-up has a couple advantages over the old way.
  • FreeNAS will boot faster.
    • FreeNAS is quite verbose during its start up, and combined with the slow throughput of the USB stick, booting up the pod took ages compared to today's world where SSD boot drives are taking over.
  • FreeNAS is safer.
    • FreeNAS is designed so that one boot SSD can fail while its mirror keeps you up and running. You simply pop in a new SSD, and once the mirror is rebuilt, you are protected again. So this gives you maximum uptime.  
    • PLUS your configuration is still being backed up! This could come in handy if you experience catastrophic failure of both SSDs, or the ever present risk of human error.
    • For more info on the Configuration Backup, check out our wiki.
EXISTING CUSTOMERS

For existing customers running FreeNAS that want to use the new set up discussed here, have no fear, as you can switch over and be up and running like nothing happened in about 30 minutes!

The complete process will be outlined on our wiki: HERE.

But here's a quick overview:
  1. Migrate all your config backups to somewhere not on the NAS, as we have to blow out the SSDs
  2. Follow our OS restoration procedure to install FreeNAS onto one of the SSDs.
  3. Plug in the freshly restored SSD the second SSD and the USB thumb drive. Ideally, get a separate USB stick than your current boot device just to be safe. It should be at least 4GB.
  4. Boot into FreeNAS, attach the second SSD to create a mirror and then create a pool called “backup” using the USB thumb drive.
  5. Import your storage pool(s) and then upload your old configuration.
  6. Get back to work!
Thank you for continually providing feedback on how we can improve our products for you, our customers. We take pride in listening to our customers, so if you have any other suggestions on how we can make our products more efficient for you, please don't hesitate to send us an email

Thursday, February 18, 2016

Guest Post- "Finding the Right NAS Operating System - If There is Just One"



Thomas Kay founded Tom's Computer Support in 2001 with the goal of providing the type of professional IT support larger companies enjoy to small businesses in Pennsylvania.  Applying his experience working in marketing, sales, as well as IT at many local companies, Thomas quickly built a reputation as being a knowledgeable and trustworthy IT industry expert. He shares his expertise on a range of current IT topics via his blog at tomscomputersupport.com. Watch this space over the next few weeks as Thomas chronicles his experience building out his NAS using a Storinator from 45 Drives.




GUEST POST: Thomas Kay's first blog post explained how his new 45 Drives Storinator is more than capable of solving the painful problem of storage expansion. In his latest blog post , Thomas discusses the options for finding the right NAS operating system for his new 45 Drives hardware. Check out the article that gives an in-depth look at the criteria involved with choosing the right NAS OS and a look at the pros and cons between using the different OS options that are available.

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Thursday, January 28, 2016

Guest Post - "The Problem of Storage"

Thomas Kay founded Tom's Computer Support in 2001 with the goal of providing the type of professional IT support larger companies enjoy to small businesses in Pennsylvania.  Applying his experience working in marketing, sales, as well as IT at many local companies, Thomas quickly built a reputation as being a knowledgeable and trustworthy IT industry expert. He shares his expertise on a range of current IT topics via his blog at tomscomputersupport.com. Watch this space over the next few weeks as Thomas chronicles his experience building out his NAS using a Storinator from 45 Drives.



GUEST POST: Thomas Kay, founder of Tom's Computer Support, brings his expertise to the 45 Drives Blog by discussing the challenge IT professionals face when balancing the accessibility of content for our users, while also managing the storage needs of the organization in a manner that is fiscally responsible. In his first article, Thomas discusses how the 45 Drives Storinator solves this painful problem of storage expansion.

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Wednesday, November 11, 2015

How to Decide on the Best RAID Configuration For You



A common question we get asked here at 45 Drives is, “What RAID should I use with my Storinator?”

Our answer: “What are you trying to do?”

Choosing which RAID level is right for your application requires some thought into what is most important for your storage solution. Is it performance, redundancy or storage efficiency? In other words, do you need speed, safety or the most space possible?

This post will briefly describe common configurations and how each can meet the criteria mentioned above. Please note I will discuss RAID levels as they are defined by Linux software RAID “mdadm”. For other implementations, such ZFS RAID, the majority of this post will hold true; however, there are some differences when you dig into the details. These will be addressed in a post to come! In the meantime, check out the RAIDZ section of our configuration page for more information.

Tuesday, September 29, 2015

Why I Love Rockstor on our Storinators



As a NAS operating system, FreeNAS has been out there for a decade. Working on the beautiful marriage of FreeBSD and ZFS, it's been tried and tested by many users within the storage community, to great success. But if you're one of those people who is more into Linux and Btrfs, you may be yearning for an alternative.

Over the past few months here in the 45 Drives lab, we have been putting an exciting new NAS OS through its paces on our Storinators. After working through a few kinks, such as how to set up redundant boot drives, we are ready to introduce Rockstor as one of our recommended solutions to create a solid NAS appliance when installed on a Storinator (alongside the traditional OS option, FreeNAS). I am writing this post to talk about why I like Rockstor, its Linux+Btrfs roots, and to share a bit of the story of how we got here.

Welcome, Rockstor.

Some of you may already be familiar with, or at least have heard of, Rockstor, a Linux + Btrfs-powered NAS OS. I, and the rest of the 45 Drives team, first heard of Rockstor when founder and lead engineer Suman Chakravartula contacted us back in November 2014 as a prospective hardware partner. We were pretty excited with the idea of helping a new NAS appliance gets its footing – it’s just another example of how the storage community really is a collaborative environment, and one that we’re proud to be a part of.