Why aren’t optical disks the top choice for archive storage?

Optical media is the longest lasting medium currently in production. It can reliably hold onto your data for 50-100 years without power or cooling, and without the worry of magnetic degradation. 

Using recordable optical media such as DVD-R is perfectly suitable for long-term archiving because it is write-once, read-many, meaning it is physically immutable—cannot be changed—so the data on it is tamper-proof.

It seems, then, that optical media might dominate archived storage, but it doesn’t. To explore why, first let’s take a look at the technology.

How optical storage works

Optical storage systems use optical disks like Blu-Ray that include a layer of reflective material. Optical drives use a laser to physically create unreflective spots called pits in an adjacent coating that can be detected by the laser that reads them. The pattern of pits and the unburned, reflective areas called lands encode the data being stored. It is physically impossible to re-burn an already-burned write-once optical disk, leading some to say optical is the only truly immutable storage option, as it cannot be changed even if someone hacked a drive.

With magnetic storage media—hard drives and tape—the magnetically stored data degrades, causing bit rot, which undermines the accuracy of the data. By contrast, the pits and lands on optical drives do not change over time and therefore have a very long shelf life without worry about bit rot. This is why makers of optical media advertise that it can hold onto data for 50 to 100 years.

Blu-ray vs optical disk

There are two types of optical media in use today. The most popular by far is Blu-ray Disc Recordable (BD-R), which can write at 72MB/s and read at 54MB/s. It also supports random access like a disk drive, so single file retrievals are very quick. BD-R drives are available as internal or external drives, and BD-R libraries are available with one or more drives and dozens-to-hundreds of slots for BD-R media. There are two main manufacturers of such libraries: HIT and DISC.

One downside to BD-R is that read/write speeds are governed by how quickly you can transfer data to and from a single optical disk. Sony worked around this limitation by putting several optical disks in single cartridge that are written to simultaneously, very similar to the way magnetic disk drives work. This creates a virtual disk that is bigger and faster than any individual optical disk.

The Sony Optical Disc Archive (ODA) drive supports capacities up to 5.5 TB and a write speed of 187.5 MB/s (with verify on) and a read speed of 375 MB/s. They are available as standalone drives that support up to 165TN and scale by stacking modules to support 2.9PB. The downside is that at $9,000, a single standalone ODA drive is 90 times more expensive than its BD-R counterpart ($100). The media itself, though, is about a third the price of BD-R media. A 5.5 TB cartridge is $184, or roughly $.03/GB, while BD-R disks are $65 for 25 25-GB disks, roughly $.10/GB.

Optical disk interoperability

One reason for the popularity of optical media is that it supports ISO 9660, which means it works like any other removable media that you might have used. Once a formatted BD-R disk or ODA cartridge is placed in a drive, it appears in the operating system and behaves like any other external drive. This allows you to copy using standard utilities and read the files in any other system that has a matching drive and the appropriate drivers.

Optical media is also designed with full backwards compatibility, meaning future BD-R and ODA drives will be able to read disks written in today’s drives. For example, you can read a CD-R disk written in 1991 in a current BD-R drive. In contrast, LTO-8 tape drives cannot read LTO-5 tape although they can read LTO-6 tapes.

BD-R drives advertise a lifetime of 50 years and Sony advertises 100 years, both of which are longer than tape (30 years) and magnetic hard drives (five years).

If you wanted a 50-year archive on LTO, you would be forced to migrate data at least once to avoid bit rot but not, as some optical marketing material suggests, every 10 years. Many people do this anyway to allow them to retire older tape drives and achieve greater storage density. There is also no current requirement to re-tension the tapes every so often. There is some debate about the bit error rate of optical versus tape, but that is a complex issue beyond the scope of this article.

The combination of very long media life that is not subject to magnetic degradation while maintaining backward compatibility is what makes optical such an attractive medium for long-term storage. The random-access nature of optical doesn’t hurt, either. If you had an optical library that could load a disk quickly, you could retrieve a file from an unloaded disk much quicker than doing the same thing from tape.

Why, then, if optical is so much better at long-term storage than disk or tape, does it not rule the long-term storage world?

It might come down to cost. At $79 for a 30TB LTO-8 tape cartridge, that’s more than 10 times cheaper than BD-R and ODA. And since most people never read their archives, any performance advantages of random access are completely lost on the average user. But for those who want truly immutable storage and have the budget for it, optical remains a solid option.