Most of the time when we work on our computers, the files we create are relatively small. We load the files into the RAM memory of our computer and work away.
Working with media, however, is different. Media files tend to be massive. Most often, they need to be played in real-time and they are too big to be fully stored in RAM memory.
Why am I saying "RAM memory?" Because many people use the term "memory" to mean "hard disk storage." For me, computer "memory" means RAM - that which disappears when power to the computer is turned off.
The "Data Transfer Rate" is the speed data travels between your hard disk storage and the computer, and it is measured in "bits per second"
To keep the numbers from getting too big when discussing hard disks, bps is often converted to MBps, or "megabytes per second." The conversion equation is simple: bps/8,000,000 = MBps. (Engineers sometimes use 8,388,608 instead of 8 million.) We also write MBps as MB/s.
There are three things that determine the speed of your data between the hard disk and your computer:
- How you connect your storage (USB 2, USB 3, FireWire, Thunderbolt...)
- The number of hard disks in your device (a hard drive has one disk, RAIDs have several)
- The internal hardware architecture of your storage device
CABLES AND PROTOCOLS
Changing how you connect your hard disk changes the speed of your data.
For example, USB 2 transfers data around 15 MB/s, FireWire 400 transfers data about 25 MB/s, while FireWire 800 is around 85 MB/s. If you spent money and bought an infinitely fast drive, then connected it to your computer via USB 2, your data would only travel at 15 MB/s, which is the speed limit of USB 2.
There's a lot of conversation today about the speed of SSD drives. SSD, which stands for Solid State Drive, can be very fast. But, if you attach an SSD drive using a very slow protocol - say, FireWire 400 - you'll never see the speed that SSD can deliver. The FireWire 400 protocol is too slow.
NUMBER OF HARD DRIVES
Connected internally, directly to the data bus of your computer, a single standard 3.5" hard drive delivers about 120 MB/s of data.
If we connect that drive via USB 2, FireWire 400, or FireWire 800, the connection protocol is slower than that of the drive, so we can't get the maximum speed from the drive because it is limited by the protocol.
However, other connection protocols, such as USB 3 and Thunderbolt can transfer data at much faster speeds than a single hard drive. USB 3 maxes out around 480 MB/s, while Thunderbolt can deliver up to 1.1 GB/s.
If you are attaching a single hard drive via USB 3, the fastest speed you can expect is about 120 MB/s, which is the maximum speed a single hard drive can deliver.
Here, the limitation is not the protocol, but the speed of the hard disk.
RAIDs allow us to combine multiple hard discs into a single unit. Now, we are able to combine the speeds from multiple hard drives so that the speed of the RAID is greater than the speed of a single hard drive.
At this point, the maximum speed of a RAID is the SLOWER of the sum of the speed of all the hard drives it contains, or the protocol that connects the RAID.
For example, a 2-drive RAID attached via USB 3 would transfer data at 240 MB/s (the sum of the two hard drives), while a 20-drive RAID attached via USB 3 would transfer data at 480 MB/s (the maximum transfer speed of USB 3).
SYSTEM OVERHEAD
Even when you've picked the fastest protocol, and connected a gajillion hard disks, there's one more factor that determines overall data transfer rate: system overhead.
This gets really complicated really quickly, however, the short answer is that every hard disk, RAID, and protocol needs to process the data both before it is sent and after it is received. And this processing takes time.
Some manufacturers opt for the fastest data transfer speeds, but sacrifice flexibility. Others opt for greater flexibility, but sacrifice speed. Some do their data processing in hardware, others use software. Some opt for maximum speed, others for data security. It is almost impossible to figure out in advance how much performance will be lost due to system overhead.
For this reason, always be skeptical when a storage vendor describes their speeds using phrases like: "Up to 480 MB/s," or "Up to 1.1 GB/s." They are quoting the speed of the protocol, not the speed of their device.
ACCURATE ESTIMATES
The easiest way to estimate the speed of a hard drive or RAID is to multiply the number of hard drives it contains by 100 MB/s. This provides a ballpark range of the data speed to expect.
CONCLUSION
If all you are doing is editing a single stream of AVCHD video, any hard disk and protocol will be fine.
As you start to edit multiple streams of video, migrate to larger image sizes, or upgrade to more professional video formats, the speed of your storage system becomes critical.
Here are some simple rules:
- A single hard drive can deliver about 120 MB/s of data
- USB 2, FireWire 400, and FireWire 800 are slower than a single drive
- USB 3 and Thunderbolt are faster than a single drive
- If your computer supports plug-in cards, eSATA and mini-SAS are also very fast protocols
- RAIDs are always faster than single drives
- RAID speeds are, generally, the sum of the speeds of the drives they contain
Here's an article that can help you learn the differences between RAIDs, SSDs, and The Cloud.
Larry Jordan is a producer, director, author, editor, and Apple Certified Trainer with more than 35 years of professional experience. Based in Los Angeles, he is also a member of the Directors Guild of America and the Producers Guild of America, and author of eight books on Final Cut Pro and Adobe Premiere Pro. Visit his website at http://www.larryjordan.biz.