Category Archives: Fundamentals

Fundamentals of Storage Systems – The System Bus

This installment we will cover what connects the controller to the computer.

Disk controllers use a system bus to talk to your CPU and memory. It also determines the maximum speed your disk can talk to the computer. There may be as many as six different system busses in your computer. We are only interested in the ones that directly connect your disk controllers.

The oldest bus still in general use is PCI. You can still find them in your desktop and in servers though it is really on the way out. We are only covering PCI 2.0 32 bits wide running at 33 MHz. This allows for a theoretical top speed of 133.33 MB/Sec. In reality after overhead and other limitations you end up around 86 MB/Sec throughput. A single modern disk can achieve this speed. You generally don’t see PCI disk controllers with more than 4 ports. Adding more disk controllers to a system may not yield a direct increase in performance. Even if you have multiple PCI slots, they may only actually run through a single PCI bus. Limiting your bandwidth to the system to 133.33 MB/Sec.

File:PCI Slots Digon3.JPG

Credit:Jonathan Zander

IBM, HP, and Compaq came together to standardize a faster bus for servers, specifically for disk controllers and network interface cards. PCI-X build on the PCI standard and was backwards compatible. It extended the PCI bus to 64 bits wide and a speed of 66 MHz in its initial launch. We go from 133.33 MB/Sec to 533.3 MB/Sec, a 4x improvement. The next generation brought us two more implementations, PCI-X 64 bit/100 MHz and PCI-X 64 bit/133 MHz at 800 MB/Sec and 1067 MHz respectively. This was a major step up but had several flaws. The physical size of the connector was huge. It also carried over the shortcomings of PCI. Signal noise across slots, errors could be caused by having several cards next to each other. Communication was half-duplex bidirectional, It couldn’t send and receive data at the same time. You are only as fast as the slowest card on the bus, If you had a 66 MHz card your 133 MHz card was reduced to match.



Credit: Snickerdo

We have moved on to a completely new standard, PCI Express (PCIe). Some people confuse PCI-X with PCIe, but they are completely different. The new PCIe standard was introduced in 2004 and was quickly adopted in main stream computers for video cards, but  it is a general system bus. There are several key differences between PCIe and the buses that came before it. It is a fully serial and bidirectional bus, you can have multiple cards at multiple speeds reading and writing data at the same time. It also introduced the concept of lanes. PCIe card will use between 1 to 16 lanes. Each lane in the 1.0 specification was rated at 250 MB/sec. The 2.0 specification introduced in 2007 doubled that to 500 MB/Sec. In 2011 the 3.0 Specification will double that again to 1 GB/Sec. PCIe is also rated by how many transfers a second it can handle. Measuring transfers in Gigatransfers or Megatransfers has been around for a while, though not commonly used. See Gigatransfers at Wikipedia for a better explanation. One thing to be aware of the 1.0 and 2.0 standard loose speed due to the way data is encoded on the bus. Like the PCI bus, the 250 MB/Sec is a maximum you won’t see in the real world. You will loose about 20%. The 3.0 specification reduces that to around 1.5%. The most common sizing of PCIe slots is 1x, 4x, 8x, and 16x. It is also downwards compatible so a 1x, 4x, and 8x card will all work in a 16x slot. Just because a card is physically a 16x it may be a 8x or slower internally. That applies for the slot as well, it may be a 16x slot but only operate at 8x speeds.


Credit: Snickerdo

PCI Express slots (from top to bottom: x4, x16, x1 and x16), compared to a traditional 32-bit PCI slot (bottom).

So, what does all this mean? If you have an older server, don’t use the PCI slots. Be careful with the PCI-X cards and placement. If you have PCIe you need to know if it is a 1.0 or 2.0 capable and what speed the physical connectors actually operate at.

Series To Date:

  1. Introduction
  2. The Basics of Spinning Disks
  3. The System Bus – You are here!
  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

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

The Fundamentals of Storage Systems – Introduction

At least once a year I give a large talk on disk subsystems, IO and SQL Server. It’s a ground up from the nuts and bolts of how a hard drive works through SAN’s and Solid State Disks. The reasons I give this presentation so often is it is one of the most requested topics and one of the most misunderstood. The problem often lies in the fact the DBA may not know that much about different storage systems but they do know that it is very important do their jobs. With the rise of SAN, iSCSI and other storage solutions DBA’s have less and less control over the disk system that their SQL Server relies on. It’s my goal to give them, or you, the tools they need to effectively present their needs to the storage teams hopefully without a major amount of fuss and arguments. If you know how and why it works they way it works you can make logical requests in the language that your storage folks understand.

The presentation is meant to lay the foundation that can then be built upon and expand your knowledge off all things I/O.

This article series will be slightly expanded over what my presentation normally covers, since I’m only restricted by your willingness to read what I write. It will still be a condensed version of storage systems but I’ll put up as many reference links as I can.

Series To Date:

  1. Introduction
  2. The Basics of Spinning Disks
  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
  10. Latency
  11. Solid State Storage Basics
  12. Understanding Reliability and Performance of Solid State Storage
  13. Shared Consolidated Storage Systems

Upcoming Posts :

Storage Area Networks
Network Attached Storage
SQL Server and The File System
Understanding Mean Time to Failure and Other Failure Metrics
Tools and Techniques To Monitor SQL Server and I/O

Some topics may be a single post some may span several I won’t know for sure until I get done writing them. As request come in I may try to post on specific questions, or at a minimum point you in the right direction.

Stay Tuned….