1. Hard drive selection guide
The following table is based upon the concepts discussed throughout this white paper. Please use it as a guide for selecting hard drives for your test, measurement, and control systems.
Table 1. Hard drive selection guide for test, measurement, and control systems.
2. Hard drive basics
The majority of hard drives use a rotating magnetic disk known as a platter to store data. The actuator moves the arm from the outer rim of the platter to the spindle in order to position the head over the platter where data will be written or read. When writing data, the head produces a magnetic field that magnetizes the surface of the platter to store data. The magnetic fields produced by platter can also be read by the head to retrieve data.
Figure 1. Rotating magnetic hard drives store data on a magnetic platter by producing a magnetic field with the head.
In order to increase data storage density, most hard drives contain more than one platter. The platters are stacked on top of each other. Each platter has a corresponding arm and head for writing and reading data.
Figure 2. Most hard drives contain more than one platter to increase data storage density.
Solid-state is another type of hard drive. Unlike rotating magnetic hard drives, solid-state hard drives contain no moving parts. They are based upon flash memory, which stores data electrically using transistors. We will discuss use cases for solid-state hard drives later in the white paper.
The condition that has the biggest impact on the life of a hard drive is temperature. Heat decreases the life of the hard drive head. A 5 °C increase in temperature could reduce the life of a hard drive by up to two years. Heat also reduces the fly height of the hard drive head, which can cause the head to make contact with and damage the media. If your system will be operated in an environment with a minimum ambient temperature less than 5 °C and/or a maximum ambient temperature greater than 50 °C, you must select a hard drive with an extended operating temperature range. These hard drives include components designed for reliability in low and high temperature extremes.
National Instruments offers extended-temperature versions of PXI and PXI Express embedded controllers that include hard drives with temperature specifications that exceed those of standard commercial hard drives (some of these extended-temperature hard drives are also extended-operation or 24/7). These hard drives have operating-temperature specifications of at least -20 to 80 °C. The extended-temperature embedded controllers have an operating-temperature specification of 0 to 55 °C and a storage-temperature specification of -40 to 85 °C.
4. Shock and vibration
Using a standard rotating magnetic hard drive may not be appropriate if your test, measurement, or control system will operate in an environment with high levels of shock and/or vibration. High levels of shock and/or vibration can cause the hard drive head to make contact with and damage the media. Because they are designed for use in laptop PCs, 2.5” hard drives are more rugged than 3.5” hard drives, which are designed for use in desktop PCs. This is because laptop PCs are typically subjected to more movement than desktop PCs.
Solid-state (flash) hard drives do not contain any moving parts, so they can tolerate exposure to higher levels of shock and vibration than standard rotating magnetic hard drives. You should consider using a solid-state hard drive if your system will be exposed to higher levels of shock and/or vibration than a standard laptop PC can withstand. You should also consider using a solid-state hard drive if your system will be exposed to shock and/or vibration when data is being written to or read from the hard drive.
It is important to realize that solid-state hard drives are significantly more expensive than standard rotating magnetic hard drives. Additionally, care must be taken when selecting a solid-state hard drive. It is feasible to have a solid-state hard drive that runs for 24 hours a day, 7 days a week sustaining 60 MB/s throughput that lasts for 7 years, but it is also feasible to have a solid-state hard drive that runs for 24 hours a day, 7 days a week sustaining 60 MB/s throughput that lasts for 0.7 years. Carefully review the documentation for the solid-state hard drives you are considering, and consult with the solid-state hard drive manufacturers.
National Instruments can upgrade your new PXI or PXI Express embedded controller with a solid-state hard drive.
5. Extended operation
If your test, measurement, or control system will be deployed into an application that requires extended operation up to 24 hours a day, 7 days a week, it is important that you understand that standard hard drives are typically designed to be powered on for 8 hours a day, 5 days a week. Additionally, your application may or may not subject the hard drive to a high duty cycle. The duty cycle of a hard drive is the percentage of maximum sustained throughput. A standard hard drive is designed for a 20% duty cycle. Applications that require extended operation and/or a high duty cycle require a hard drive specifically designed for these operating conditions and are often referred to as “24/7” hard drives.
National Instruments offers extended-operation versions of PXI and PXI Express embedded controllers that include 24/7 hard drives. These 24/7 hard drives are also extended-temperature.
In addition to selecting an appropriate hard drive, using adequate system memory (or RAM) is also important if your system will be deployed into an application that requires extended operation. If the programs running on your system require more memory than is installed, the operating system will use the hard drive as virtual memory. This is referred to as hard drive paging, and if it takes place continuously, the hard drive can experience a decreased life. For applications that require continuous operation up to 24 hours a day, 7 days a week, you should verify that your system has enough memory to fulfill the requirements of your software.
A number of specifications dictate the overall performance of a hard drive. These specifications also determine the sustained rate at which data can be written to or read from a hard drive, which is often referred to as streaming. Three of the most influential specifications are seek times, buffer size, and rotational speed.
Four different seek times are specified for a hard drive – read seek time, write seek time, track-to-track seek time, and full stroke seek. Each specifies the time it takes for the hard drive to position the head for a particular operation. Seek times are specified in milliseconds, and lower values indicate higher performance.
Hard drives include a small amount of onboard RAM that provides a high-speed buffer between the interface and the media. A larger buffer improves performance because more data can be written to and is available to read from this high-speed location. Data that is frequently accessed can be stored in the buffer. Also, if the hard drive media is not able to keep up with the rate at which data is being written by the interface, the buffer is available to store data. Buffers typically range in size from 2 MB to 16 MB.
The specification that has the biggest impact on overall hard drive performance, and sustained streaming rates, is rotational speed, which is specified in RPMs. The faster the rotational speed, the higher the rate at which data can be written to or read from the hard drive. Common rotational speeds are 4200, 5400, and 7200 RPM. Performance hard drives are available with rotational speeds of 10000 RPM.
A typical 2.5” hard drive can sustain writing or reading at a rate of 15-30 MB/s. A typical 3.5” hard drive can sustain writing or reading at 20-65 MB/s. One way to increase this rate is to use a RAID-0 array of multiple hard drives. RAID stands for redundant array of independent disks. A RAID-0 (striped) array increases the rate at which data is written to and read from hard drives by evenly distributing data among multiple hard drives. Using an External SATA (eSATA) ExpressCard in the ExpressCard slot that is available on many PXI and PXI Express embedded controllers from National Instruments, data can be streamed to or from a RAID-0 hard drive array at over 100 MB/s. Rack-mount PXI and PXI Express controllers from NI are also available preconfigured with a RAID-0 hard drive array.
Figure 3. A RAID-0 (striped) array increases the rate at which data is written to and read from hard drives by evenly distributing data among multiple hard drives.
Using a RAID configuration that includes redundancy enables your test, measurement, or control system to recover from a hard drive(s) failure. With a RAID-1 (mirrored) hard drive configuration, each piece of data is written to two (or more) hard drives. In a two-drive RAID-1 configuration, if one hard drive fails, the system can continue operating using the functional hard drive without any system downtime. PXI and PXI Express rack-mount controllers from National Instruments can be configured with two-drive RAID-1 arrays.
Figure 4. A RAID-1 (mirrored) array provides redundancy by writing each piece of data to two (or more) hard drives.
8. Software recovery and backup
If you encounter a software issue with your test, measurement, or control system, being able to take advantage of recovery software can reduce downtime, sometimes significantly. Recovery software enables you to recover your system back to a known-good state. Many commercial recovery software packages are available.
All National Instruments Windows-based PXI and PXI Express embedded and rack-mount controllers include hard drive-based recovery images and software. When you receive the controller from NI, it has a separate partition on the hard drive that includes a complete back-up image. At any time, you can re-image your controller back to this original image without needing a CD, secondary hard drive, network connection, etc. Moreover, you initiate the recovery process during boot, before Windows has loaded, so even if your controller cannot boot into Windows, you can still recover back to the original image.
The recovery software that is included on PXI and PXI Express embedded and rack-mount controllers from NI also gives you the ability to create custom backup images that can be stored to external or secondary hard drives, such as USB, and used as a custom recovery image instead of the original NI image. After your system is completely assembled and tested, you can create a custom backup image to use for recovery if you experience software problems after deployment.
Figure 5. Recovery software enables you to recover your system back to a known-good state and reduce downtime.
9. Diagnosing hard drive issues
Many test, measurement, and control system problems are misdiagnosed as faulty hard drives when the actual problem is due to software, file corruption, file system corruption, or a virus. Most hard drive vendors provide hard drive diagnostic tools that determine if an actual hard drive failure has occurred. These tools can save you costly troubleshooting time and should be used before returning the hard drive, PC, or controller for repair. National Instruments provides a hard drive diagnostic tool for use with PXI and PXI Express embedded controllers. If you use a hard drive diagnostic tool to determine that your hard drive has not failed, then you can take advantage of recovery software to fix the issue.
10. Spares and replacements
Stocking spare components for your test, measurement, or control system will significantly reduce the downtime you experience if a hardware failure occurs. A general rule of thumb is to stock one spare for every 25 systems you deploy. National Instruments offers spare/replacement hard drives for PXI and PXI Express embedded controllers. These kits include detailed documentation that explains how to replace the hard drive in the field.
11. Appendix A - Hard drive interfaces and sizes
When you begin the process of selecting a hard drive, one of the first things that you will notice is the fact that many different types of internal hard drive interfaces exist – Serial ATA (SATA), Parallel ATA (PATA), SCSI, Serial Attached SCSI (SAS), etc. SATA and PATA, because of their widespread adoption in the commercial PC market, offer the best combination of price and performance. SATA, as the names indicates, is a serial interface, and is an evolution of the parallel interface of PATA. SATA offers higher throughput, 300 MB/s for SATA/300 versus 133 MB/s for PATA/133 (these are the maximum throughputs of the interfaces and do not reflect the maximum sustainable throughputs of the hard drives), and SATA hard drives are quickly becoming the standard for both desktop and laptop PCs.
Figure 6. Serial ATA (SATA) and Parallel ATA (PATA) are the most common internal hard drive interfaces.
In addition to different interfaces, hard drives are available in different sizes. 3.5” and 2.5” hard drives are the most common. 3.5” hard drives are used in desktop PCs, and 2.5” hard drives are used in laptop PCs and embedded controllers for modular platforms, such as PXI. Both 3.5” and 2.5” hard drives are available with SATA or PATA interfaces.
Figure 7. 3.5” and 2.5” are the most common hard drive sizes.