As the economics of data storage push more and more data onto disks, the energy efficiency of data storage is ever more critical. Storage is anti-entropic, so keeping bits organized requires energy. How can we minimize that energy input?

Data cooling is the major reason disk drives have remained a viable storage strategy for 50 years. The IOPS/MB has dropped steadily for decades, yet disks remain the preferred tool outside of very low latency or high-bandwidth applications.

Looking forward to massive scale-out storage infrastructures the data will get even cooler. Copan’s MAID architecture, which turns disks off when not in use, is a rational extension of the cool data concept.

As data continues to cool we will eventually see millions of disk drives – along with tapes – sitting idle. But even if we have cold archive disks, one of disk’s big advantages over tape is the ease with which data can be spread over multiple drives for data protection.

Not RAID 5
You can’t count on any one hard drive actually restarting after a few months or years of idle time. Nor can you expect that any specific sector will be readable. Cold data requires even more advanced – energy efficient, disaster-tolerant – storage techniques than RAID arrays offer today.

Oh, and they need to be cheap too. Which means RAID arrays won’t get this business. What about open source software?

Erasure coding
In A Performance Evaluation and Examination of Open-Source Erasure Coding Libraries For Storage (pdf) James S. Plank, Jianqiang Luo, Catherine D. Schuman, Lihao Xu, and Zooko Wilcox-O’Hearn examine 5 open source implementations of 5 different erasure codes: Reed-Solomon, Cauchy Reed-Solomon, Even-Odd, Row Diagonal Parity and Minimal Density RAID 6 codes.

Picture 7
Typical storage system with erasure coding – figure from the paper

Several companies – including Cleversafe, NetApp and Panasas – use erasure codes today to ensure higher data availability. What Plank et. al. wanted to know is how well these codes work and what system designers need to know to use them effectively.

The OSS implementations tested are:

  • Luby, a C version of CRS.
  • Zfec, a highly tuned Reed-Solomon library.
  • Jerasure, a GNU LGPL C library that includes RS, CRS and 3 MDR6 among others.
  • Cleversafe
  • released an open source version of their dispersed storage system, from which the authors used just erasure coding parts.

  • , patented codes not available to the public and included for performance comparison.

Most important result
The study found that while tuning boosts performance and some architectures are much faster than others,

Given the speeds of current disks, the libraries explored here perform at rates that are easily fast enough to build high performance, reliable storage systems.

Translation: this isn’t string theory.

Other findings include:

  • The RAID 6 codes out-perform the general purpose codes.
  • For non-RAID 6 codes, the Cauchy Reed-Solomon performs much better than straight RS
  • CPU architectural features, such as cache size and memory behavior, make it hard to predict an optimal data structure for a given code configuration.
  • The code’s memory and cache footprint can have a large impact on performance.
  • Specialized RAID 6 codes hold promise for creating efficient storage that can withstand numerous concurrent disk failures.
  • Multicore performance issues are largely unexplored.

The StorageMojo take
The architect for a planned commercial 200 PB cold-storage infrastructure confessed that he can see how to get to 25 PB today, but not beyond. Yet they have no choice but to start building now.

This market’s eventual structure may parallel that of today’s tape silo market: everal hundred large customers who are continuously churning through rolling upgrades of media and servers.

Right now, tape silos still enjoy an economic advantage over disks. But it looks like disks have more degrees of freedom to improve their cold storage economics than tape.

In just 5 years the first exabyte cold storage systems will be on the drawing boards. It is time for disk companies to get serious about a tape-replacing archival disk. And for clever startups to focus on this emerging market.

Courteous comments welcome, of course.