Data Center Advisors

In March of last year, I wrote a blog about disruptive technologies, specifically how the Blu-ray technology could change the storage landscape (or not). The new Blu-ray disc format specification enhancement was defined in June 2010 for the BDXL standard. This new specification increases the recording capacities for Blu-ray discs in two ways.

First, per-layer formatted capacity has increased from the initial 25GB to about 33GB per layer. Second, the number of layers now supported on a disc is three and four, or stated as a triple and quad layered disc. The conventional capacity description for the triple layered disc is now 100GB (99+GB) and 128GB per disc. There is some discussion with Blu-ray media suppliers looking for requirements for dual sided media, which could increase the per disc capacities to 200GB for a six layered disc (triple layers on both sides) and a 256GB for an eight layered disc (quad layers on both sides).

Now, when I discuss optical storage capacities and Blu-ray, I’m typically referring to the storage capacities and read/write speeds for these devices, and how they would apply to enterprise uses. I’m less interested in the video formatting and supported formats—at least until I’m working on my own home videos. For my job at HDS, I’m always watching this space for the “kicker” that sends this technology to the forefront of an enterprise’s alternative storage technology strategy. More specifically, when could optical storage technologies, like Blu-ray, replace tape (albeit, for certain types of use cases)?

Even with an inconveniently laid out 256GB of capacity (flipping a disc over is a pain at best), Blu-ray would have a tough time replacing the current multi-terabyte LTO tape formats today and planned improvements for the future.

But wait! What’s actually being compared when discussing LTO tapes and Blu-ray discs, head-to-head on capacity, or head-to-head on capacity and footprint? The current specifications for an LTO5 tape is 1,500GB of uncompressed capacity with a cartridge dimension of 4.1 x 4.0 x 0.8 inches (105 x 102 x 21 mm). Using some creative math, this is 13.12 cubic inches, which works out to about 114.3GB per cubic inch by that cartridge.

The specifications for a quad layered disc (single sided) is 128GB of uncompressed capacity with a disc dimension of 4.7 inches in diameter x 0.047 inches thick (120 mm diameter x 1.2 mm thick). Again, doing some creative math, this is 0.815 cubic inches, which in this cases works out to 157.1GB per cubic inch. This is an amazing 314.1GB per cubic inch if we test the waters with a dual sided disc. Stated another way, in roughly the same amount of space that an LTO5 cartridge occupies for 1,500GB of uncompressed storage capacity, a stack of Blu-ray discs would contain a whopping 2,060GB using a single sided 128GB quad layered disc and over 4,100GB for a stack of double-sided quad layered 256GB discs.

Granted, the Blu-ray disc is a bit wider than the LTO cartridge and a straight stacking of discs or a forced fitting of cubic inches from one form factor into the other is not a precise or practical method of comparing these two storage medias. Also, in a well designed apparatus for managing many optical discs, there would be zero surface contact required, so a measureable gap between discs would be needed to manage them properly. This would drop the GB per cubic inch capacity somewhat.  However, the numbers are disparate enough to look closer at this from a different perspective.

So what about performance? The current LTO5 specification states a 140MB/s rating for the uncompressed 1,500GB format. This means for a read operation, once the tape sequentially seeks to the requested location, at top speed (tape moving across read/write head) the data can be read at 140MB/s. Impressive and roughly as fast as most magnetic hard discs sequentially streaming today. A Blu-ray disc reads and writes at about 18MB/s using the slim form factor optical disc drives and about twice this speed for the full and half height form factor optical disc drives. So for comparison sake, I’ll use 30MB/s for read/write as a conservative estimate. This is one of Blu-ray’s main deficiencies when looking to be used in the enterprise as a serious storage alternative, poor performance, as well as, per disc capacity.

However, let’s look at this from a different perspective again. That same “stack” of discs that yields up to 2,060GB of capacity when compared to LTO5’s footprint, would roughly need 17 optical disc drives to simultaneously load all of these discs. Two things happen when this is done. First, 17 drives x 30MB/s is a total of 510MB/s of aggregate performance from this “stack” of discs. This assumes reading ALL of the discs for some sort of read everything or write everything operation. Second, even if there isn’t this many optical disc drives available, all 2,060GB of data is divided into 128GB chunks and 33GB layers. When seeking a single file from a Blu-ray disc, the operation would only load the disc with the requested file on it, focus the laser to the required layer, then directly seek to the location on that layer to read the file and satisfy the request.

Of course both storage mediums have excellent power consumption characteristics when not in use – ZERO watts. However, the knock on optical storage (among many others), specifically Blu-ray, I think in many cases is unfairly compared (myself included initially). Individual medium – LTO5 cartridge – to individual medium – disc – is the standard comparison, but both will be used and managed in similar ways. That is, nobody uses just one, and there’s always some device in place to manage them as a larger body of media. This means when the aggregation of capacity and performance is measured, the two technologies compare fairly well, at least now that the new BDXL format is being used for Blu-ray.

I would like to hear about your experiences with Blu-ray storage and what your opinions are concerning this technology–especially those professionals using Blu-ray in their companies to solve problems.