IVS 2000 General Meeting Proceedings
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Haystack Observatory Technology Development Center

Alan Whitney

MIT Haystack Observatory

Abstract:

Current work in VLBI technology at Haystack Observatory includes further development of the Mark IV correlator system, plus a new initiative to develop a low-cost high-performance Mark V VLBI record/playback system, that will utilize mostly commercial-off-the-shelf (COTS) technology and most likely incorporate magnetic discs as the storage media.

1. Mark IV Correlator

Mark IV correlators have been operational at USNO, MPI, JIVE and Haystack for about a year and have now completely replaced the Mark IIIA correlators. Haystack Observatory maintains the software for the USNO, MPI and Haystack installations.

Before the changeover from the Mark IIIA correlators to the Mark IV correlators, extensive cross comparison tests were done to ensure that the Mark IV results were of the highest integrity. In fact, the first published results from the Mark IV correlators occurred less than 2 months after the changeover from the Mark IIIA correlator.

The Mark IV correlator software has been continuously improved over the past year to include new operational capabilities and efficiency improvements, and all Mark IV correlators are now operating at an efficiency exceeding that of the Mark IIIA, with still more improvements to be made. Some of the features that are now supported are:

Soon to be implemented will be:

2. Mark V VLBI Data System Development

With support from NASA, JIVE and NRAO, Haystack Observatory is now developing the Mark V VLBI record/playback system based on the requirements for the next generation of VLBI systems:
Though this list of goals may appear idealistic, we believe they can be achieved in a system which can be designed today, and which will evolve in the following years to even higher data rates and additional storage capacity with minimal additional engineering effort.

2.1 Magnetic discs on the way to surpassing tape

Though both magnetic disc technology and magnetic tape technology have made great strides over the past few years, the pace of magnetic disc development has been so great that it is very likely that disc storage will become cheaper than magnetic tape storage by $\sim$ 2004. This trend can be clearly seen in Figure 1, where the $/GB for both disc drives and magnetic tape media (including projections for LTO tape media, which we focused on in the original COTS concept proposal) are plotted as a function of time. In addition are shown two 1998 projections for disc- storage costs, one from IBM and the other from the National Storage Industry Consortium (NSIC); the latter suggests a tape-to-disc crossover in the latter part of this decade. However, it is clear that the cost decreases of magnetic disc storage have dramatically accelerated over the past two years, far exceeding industry projections of only 3 years ago, and show no signs of ceasing. Current disc industry predictions suggest that in 2004 the cost per GB will be $\sim$ $.30 for low-cost `dimestore' IDE (aka ATA) drives, but even these predictions may well prove conservative. As you can see from Figure 1, by $\sim$ 2004-5 the cost of disc drive storage is expected to fall below that of magnetic tape. Magnetic tape industry predictions have historically been pretty much on target and are expected to continue to be so, provided magnetic tape can even survive in the face of magnetic disc progress! Furthermore, note that the costs shown in Figure 1 are for complete disc drive units, while the tape costs are for the magnetic tape media, ignoring the substantial cost of tape drives.

The reason for this happening is quite clear - the flow of money into magnetic disc development is perhaps two orders of magnitude greater than that for magnetic tape! The disc industry sees no apparent obstacles to the current breakneck pace of development for at least several more years. Magnetic disc area-bit-densities on modern discs now (in early 2001) are $\sim$ 15 Gb/$in^{2}$, and are expected to rise to at least $\sim$ 100 Gb/$in^{2}$ by 2004. This is even far outstripping optical recording technology, which appeared to have such a bright future just a few years ago but has not become a serious competitor to magnetic discs for high-data-rate, high-density storage. We believe the VLBI community should begin to prepare now for a changeover to magnetic discs, which is the front-runner technology for the new Haystack Mark V system.

 
Figure 1: Disc/Tape Price Comparisons
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2.2 The Mark V VLBI Data System

A `Mark V' VLBI data system based on magnetic disc technology is now being designed at Haystack Observatory. Based on a standard PC platform with standard commercial busses and interfaces, it will support a continuous data flow of at least 1 Gb/sec to an array of up to $\sim$16 magnetic discs. The only custom hardware will be VSI-H formatter and deformatter interfaces. Because the Mark V system is based on a standard PC platform, the data stored to discs will also be available for local analysis by the PC or for transmission over a network for e-VLBI. The cost of either the 1 Gb/sec Mark V recording or playback system (without discs) is expected to be $<$$25K. Multi-Gb/sec systems will be constructed simply by using Mark V systems in parallel, though future versions of the Mark V may have standalone multi-Gb/sec capability. The Mark V system will be explicitly based on disc drives with IDE interfaces, which are the most cost-effective consumer discs that can be procured. Furthermore, the IDE interface is so pervasive and so successful that any successful follow-on to magnetic discs, such as (possibly) optical discs, will almost certainly also be available with IDE interfaces, in which case they can be adapted to the Mark V system almost seamlessly. It is important to note that the Mark V data system is only a data recording and playback system and will require the use of a separate set of back-end electronics (IF converters, BBC's, samplers, etc).

2.3 The 24-hour Challenge

Disc industry expectations are that in 2004 a terabyte disc will be available for $\sim$ $300. The recording of a continuous data rate of 1 Gb/sec for 24 hours will consume $\sim$ 11 TB of storage, so that an array of only $\sim$ 12 such discs for a total cost of $3600 will comfortably operate unattended at 1 Gb/sec for 24 hours. This compares to today's cost of $>$$20K for VLBA/Mark IV media of the same storage capacity.

Disc drives will be mounted in carriers, holding either single or multiple discs, made for multiple insertion/removal cycles. When modern disc drives are powered down, they are quite robust to external handling forces and can be shipped easily in padded containers. Including the carriers, the shipping weight per disc should be $<$$\sim$1.5 kg, so that the shipping weight of 12 discs containing an aggregrate of 11 TB will be $<$ $\sim$18 kg.

2.4 Other Benefits of Disc-Based Mark V VLBI Data System

There are a number of other obvious benefits of magnetic discs over magnetic tape:

2.5 Mark V Development Schedule

Haystack is now beginning the development of the Mark V system based on magnetic disc technology. Early demonstrations of feasibility may occur as early as the end of 2001, with a deployable system ready as soon as $\sim$ 2003.

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IVS 2000 General Meeting Proceedings