Following a number of video editing forums, the question of what kind of disk should be used, often comes up. I’ve seen this general question come up a lot on the forums — “Is my disk OK? What kind of disk do I need?” There are a lot of good answers to these questions that get people working quickly, but I thought I’d take a little bit and explain a bit more of the “why” and “how” this all works. I’m going to keep this at somewhat of a high level and limited to what we would typically put in our Macs.
Fundamentally, you are concerned with two basic properties of the system:
- How fast you can get data to/from the disk
- How fast the disk itself can read/write that data
First, how fast can we get data to/from the disk. This is governed by the interconnect technology used to connect the CPU/Memory to the disk. On a typical Mac, you have a few options:
- SATA (Serial ATA): This is what your internal hard drives are connected with. This can nominally push data at 3 Gbits/s (3072 Mbits/sec). SATA was designed for hard drives.
- eSATA: Simply SATA with special connectors to support external disk enclosures.
- USB: This will nominally push data at 480 Mbits/sec. Due to the way USB works, it can have performance issues keeping up performance transferring large amounts of data — expect to see half or less of the nominal rate. USB was designed as a general peripheral connection technology — not necessarily optimized for disks storage.
- Firewire (IEEE 1394): Comes in two versions, FW 400 (nominally 400 MBits/s) and FW 800 (800 MBits/s). Firewire was designed to support disk storage along with other audio/visual connectivity protocols.
The disks themselves… So what are the important parameters to look for in the disk itself:
- Rotational Speed: This is how fast the drive spins. You will typically see two different speeds 5400 rpm and 7200 rpm. There are increasingly “green” drives appearing on the market which I’ll discuss briefly later. Hard drives have spinning platters (disks) and a set of read/write heads inside them. This isn’t much different than a phonograph with the disks being the vinyl record and the read/write heads being the needle. To access a bit of data, the heads have to move to a specific radial position and then wait until the write spot is spun underneath them. Remember that data (your files) is, to various degrees spread all over the disks. When you read or write a lot of data the heads are constantly repositioning and waiting as right data passes under them. Think of trying to listen to several songs at the same time on a vinyl record and you get the idea.
- Cache: Disks have cache on them that can speed up read and write operations. Bigger is generally better, but cache is only so good.
- Data Transfer Rate: This is fundamentally a function of the rotational speed, but it tells you how fast the disk can actually read/write. Each disk has different specs, so you have to check the disk you’re considering. Also be very careful as to exactly what performance is being reported by the manufacturer and make sure you are doing apples to apples comparisons.
Now to tie it all together. You’re speed is going to be limited by the slowest part of the chain. If you put a slow disk with a fast interconnect, you’re limited by the slow disk and vice versa. In practice, the slowest options of the above are USB and 5400 rpm drives. Either SATA or Firewire has a higher data rate than the 7200 rpm disk can sustain. The other thing to consider is how many concurrent IO operations you are sending to the disk. Your system disk (Macintosh HD) has to service all of the other IO needs of your system. If you add your video editing demands to this, you could overwhelm the drive.
I think there are a few common problems that people run into:
- The MacBook Pro’s standard drive is a 5400 rpm disk. This is probably going to be too slow for most video editing demands, especially when you add additional IO demands. It will “work” but you probably won’t be happy.
- Disk is too full. You need to leave 10% – 20% of your disk free. The OS needs this free space to easily organize the data it’s writing to disk.
- Just sending too much IO to a single disk. A single disk can only handle so much IO. Once you go over that, you wait (or drop frames).
The most common solution is to use a 7200 rpm disk connected by Firewire. 7200 rpm because it’s the fastest you’re going to reasonably afford and Firewire because it’s fast enough and every Mac (maybe not the MacBook) has it. If you have a MacPro, you can look at adding more internal SATA drives (consider RAID’ing them for more performance) or look at eSATA (need to buy a controller in addition to disks).
If you’re in a bigger setting with shared storage arrays, you have even more options and things get even more complicated. I’ll not going into that here, but am happy to discuss in more detail if anyone is curious.
P.S. What about those “Green” disks — I want to be nice to the environment. The Green drives that are becoming popular work by dynamically changing speeds based on demands, with their primary goal to save power and thus run at slower speeds. When you start sending data to these disks, they typically have to spin up from some nominal lower speed to something faster to support your IO. You pay a penalty (wait) for the spin up and are at the mercy of the drive to decide if it wants to spin up to full speed. These are great for longer term storage for data infrequently used (say a Time Machine disk, Aperture Vaults, iTunes music collection, or just online archive storage of files (vidoe or otherwise)) but not for something needing consistent, fast throughput, say Video Editing.
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