Category Archives: Hard Disks

Changing Directions

I See Dead Tech….

Knowing when a technology is dying is always a good skill to have. Like most of my generation we weren’t the first on the computer scene but lived through several of it’s more painful transitions. As a college student I was forced to learn antiquated technologies and languages. I had to take a semester of COBOL. I also had to take two years of assembler for the IBM 390 mainframe and another year of assembler for the x86 focused on the i386 when the Pentium was already on the market. Again and again I’ve been forced to invest time in dying technologies. Well not any more!

Hard drives are dead LONG LIVE SOLID STATE!

I set the data on a delicate rinse cycle

I’m done with spinning disks. Since IBM invented them in nineteen and fifty seven they haven’t improved much over the years. They got smaller and faster yes but they never got sexier than the original. I mean, my mom was born in the fifties, I don’t want to be associated with something that old and way uncool. Wouldn’t you much rather have something at least invented in the modern age in your state of the art server?

Don’t you want the new hotness?

I mean seriously, isn’t this much cooler? I’m not building any new servers or desktop systems unless they are sporting flash drives. But don’t think this will last. You must stay vigilant, NAND flash won’t age like a fine wine ether. There will be something new in a few years and you must be willing to spend whatever it takes to deploy the “solid state killer” when it comes out.

Tell Gandpa Relational is Soooo last century

The relational model was developed by Dr. EF Codd while at IBM in 1970, two years before I was born. Using some fancy math called tuple calculus he proved that the relational model was better at seeking data on these new “hard drives” that IBM had laying around. That later tuned into relational algebra that is used today. Holy cow! I hated algebra AND calculus in high school why would I want to work with that crap now?

NoSQL Is The Future!

PhD’s, all neck ties and crazy gray hair.

Internet Scale, web 2.0 has a much better haircut.

In this new fast paced world of web 2.0 and databases that have to go all the way to Internet scale, the old crusty relational databases just can’t hang. Enter, NoSQL! I know that NoSQL covers a lot of different technologies, but some of the core things they do very well is scale up to millions of users and I need to scale that high. They do this by side stepping things like relationships, transactions and verified writes to disk. This makes them blazingly fast! Plus, I don’t have to learn any SQL languages, I can stay with what I love best javascript and JSON. Personally, I think MongoDB is the best of the bunch they don’t have a ton of fancy PhD’s, they are getting it done in the real world! Hey, they have a Success Engineer for crying out loud!!! Plus if you are using Ruby, Python, Erlang or any other real Web 2.0 language it just works out of the box. Don’t flame me about your NoSQL solution and why it is better, I just don’t care. I’m gearing up to hit all the major NoSQL conferences this year and canceling all my SQL Server related stuff. So long PASS Summit, no more hanging out with people obsessed with outdated skills.

Head in the CLOUD

Racks and Racks of Spaghetti photo by: Andrew McKaskill

Do you want this to manage?

Or this?

With all that said, I probably won’t be building to many more servers anyway. There is a new way of getting your data and servers without the hassle of buying hardware and securing it, THE CLOUD!

“Cloud computing is computation, software, data access, and storage services that do not require end-user knowledge of the physical location and configuration of the system that delivers the services. Parallels to this concept can be drawn with the electricity grid where end-users consume power resources without any necessary understanding of the component devices in the grid required to provide the service.”

Now that’s what I’m talking about! I just plug in my code and out comes money. I don’t need to know how it all works on the back end. I’m all about convenient, on-demand network access to a shared pool of configurable computing resources. You know, kind of like when I was at college and sent my program to a sysadmin to get a time slice on the mainframe. I don’t need to know the details just run my program. Heck, I can even have a private cloud connected to other public and private clouds to make up The Intercloud(tm). Now that is sexy!

To my new ends I will be closing this blog and starting up to document my new jersey, I’ll only be posting once a year though, on April 1st.

See you next year!

Fundamentals of Storage Systems – The Basics of Spinning Disks

Your servers are only as fast as the slowest part, hard drives.To feed other parts of the system we have to add lots of drives to get the desired IO single server can consume.

The basics of how hard drives work has been fundamentally static since the 70’s only refinements in technique and the core technologies have improved. You have a shaft or “spindle” attached to a motor. Disks or “platters” are attached to the spindle. The motor spins the spindle and the platters. Read/write heads controlled by actuator motors move across the surface with very precise motion and access the information stored on the platters. Generally, there is one read/write head per platter surface that is useable.


This configuration has worked so well for the last 45 years that every claim to date that X new technology would unseat it just hasn’t happened. That’s not to say it won’t happen, just that hard drives have been “good enough” for the bulk of our storage needs for a very, very long time. Since this is the core of our permanent storage in our database world it is important to have a basic understanding of them.


Six hard disk drives with cases opened showing platters and heads; 8, 5.25, 3.5, 2.5, 1.8, and 1 inch disk diameters are represented.
1 March 2008(2008-03-01)
Paul R. Potts

I love this picture. Smaller and faster yet still the same.

To give you an idea of what you are up against lets compare the growth rate of your hard drive VS. your CPU.

Our 1981 machine has a the veteran Seagate ST-412 and a Intel 8088.
Our new computer has a Seagate Cheetah 15k.6 ST3146356SS and a Core i7 965 from Intel of course.

Time Circa 1981 Today Improvement
Capacity 10MB 1470MB 147x (209715x for 2TB drive)
Seek speed 85ms 3.4ms 20x (6x for 2TB drive)
IO/Sec 11.4 303 26x
Mbit/Sec 5 (0.625 MB/Sec) 1000 (125 MB/Sec) 200x
CPU 4.77Mhz(.33 MIPS) 3200Mhz(18322 MIPS) 5521x

At first glance we can say WOW what an improvement! Right up until you see how far the processors have come.Everyone is familiar with Moore’s law (Often quoted, rarely understood) loosely applied says CPU transistor counts double roughly every 18 to 24 months.Up until recently, hard drive capacity has been growing almost at the same rate doubling in size around every 18 months (Kryder’s Law).  Hard disks haven’t come close to keeping up with that pace, performance wise. Again, the problem isn’t size is speed.

The Makeup of A Modern Hard Drive

You cannot change the laws of physics” – Scotty

As I stated in the previous section hard drives have remained relatively unchanged since the IBM Winchester drive. Lets take a closer look at the physical structure.

Head, Sectors and Cylinders

So, we have a spindle one or more platters and one or more read/write head, all of that spinning and jittering about at a pretty good clip. So, just how does the computer know where your data is? The platter is broken up into a map of sorts.

Simplistic view:


The platter is broken up into concentric rings and pie slices that allow the drive controller to find the region where the data is.


The heads all move in unison and present a view through the platters that make up a cylinder. I won’t go into great detail on how we have advanced sector and track layouts and the advent of Logical Block Addressing there are plenty of articles on the web that get into those nuts and bolts. What I’m after is to show you physically what has to happen to read the data from the disk and why that is the limiting factor. With the disk spinning at 15,000 RPM the sectors are flying by pretty quickly so the head has to be positioned above the sector and then read or write to it as the platter moves underneath it. The spinning disk, moving the heads and waiting for the data to be read all add up to latency.

Rotational latency is how long it takes the sector we are after to move under the head to be read or written to. Average rotational latency is expressed as half the time it takes for the platter to make one revolution. For our 15k hard drive that number is 2 milliseconds, 60 seconds divided by 15000 RPM divided by 2.

Seek Time is how quickly the disk head can be positioned over a sector to start reading data.

There are to kinds of seek we are interested in, average random seek time and sequential or track-to-track seek times.

In our top of the line Seagate Cheetah our random read seek time is 3.4ms that is the time it takes to get from any one sector to any other sector, usually half the distance from the inner track to the outer most track. Random write seek time is 3.9ms. It is longer due to the process of actually effecting the sector its at before moving on to the next random sector.

sequential is much much faster. If the head only has to move to the next track it can usually do so in under a millisecond.

All this adds up to an average access time. basically, you take the rotational latency plus the average random seek time and any command processing time overhead I usually throw in an additional millisecond. Our Cheetah has a random access time of 6.4ms. Sometimes it may be much faster sometimes it may be much slower but this is a good number to work with as far as planning our storage needs.

The flip side of operations per second is throughput usually expressed in megabytes a second.

This is a direct correlation to the amount of data that can be squeezed into a sector. As drive densities go up so does the average megabytes per second. There is something you should know, the inner tracks are slower on throughput but higher on IO’s and the outer tracks are higher on through put and lower on IO’s. This is just a function of the diameter of the platter getting larger the farther out you go.

It isn’t unusual to see sequential throughput average of around 110 MB/sec and that is only getting better.

Random throughput is not so rosy a picture. I haven’t seen any drive manufactures advertise these numbers from my own testing it can be as little as 15MB/sec up to 40MB/sec. You should test your system to get more accurate numbers.

What It All Means to Us

This boils down to how many I/O operations a single disk can give us. In SQL Server land random IO is king and generally one of the biggest bottlenecks on our data files.For log files, things are a little better. Since logs are written to sequentially you can effectively double the available I/O’s a drive can provide since you have cut our the random access and are much closer to the sequential or track-to-track access.

To calculate the maximum number of random operations we use 1000ms / (seek time[ms] + latency[ms]+overhead[ms])= input/output operations per second.


1000/(3.4+2+1) = 155 IOps

Sequential reads get much better since seek times go down from 3.4 to around 0.2.

1000(.7+2+1) = 270 IOps

Almost twice as much! Now you know why we keep our database log files separate from each other and from the data. The amount of disks needed to get the performance is about half. We do the same thing for writes and they will be a little less.

Hard drives suffer from what is known as the “hockey stick” effect the closer they get to 100% utilization the performance falls off dramatically.


Since running a disk at 100% capacity for IO’s introduces the maximum possible latency. The knee of the curve is around 80% we back that off a little more to 75% and that gives us the number of IO’s we have available per hard drive in the storage system in general. This reduces Queuing and keeps latency low, at the cost of maximum number of IO’s. Now our available read IO’s is down to about 117 IOps for random access and 216 IOps for sequential.  This number will get better as seek times get better and the command overhead gets better. But remember it will never ever be better than the 2.0ms for the rotational latency. Physics can be a real bummer sometimes. Along with physical spindle speed there have been large improvements with how the drive handles incoming and outgoing request. Through IO Prioritization and advanced command queuing algorithms (Native Command Queuing on SAS/SATA) access times and latencies are kept predictable and as fast as possible.

Series To Date:

  1. Introduction
  2. The Basics of Spinning Disks – You are here!
  3. The System Bus
  4. Disk Controllers, Host Bus Adapters and Interfaces
  5. RAID, An Introduction
  6. RAID and Hard Disk Reliability, Under The Covers
  7. Stripe Size, Block Size, and IO Patterns
  8. Capturing IO Patterns
  9. Testing IO Systems