NSSDC's primary storage system for network-accessible astrophysics and space physics data is the near-line (robotics-involved) system called NASA Data Archive and Distribution Service (NDADS), now in its sixth year of operation. Through these years NDADS's storage capability has consisted primarily of a pair of 12-inch Write-Once Read-Many (WORM) optical disk jukeboxes front-ended by a VAX computer clustered with other NSSDC computers. The capacity of NDADS was increased once in 1995 from its initial value of 1.2 TB to 1.6 TB. As of this time NSSDC has written close to 2.0 TB of data to WORM disks, with the overflow beyond NDADS near-line capacity being accommodated by off-line WORM disks and a stand-alone WORM drive acting as a software-enabled virtual jukebox.
NSSDC is about to inaugurate as a new element of NDADS a Digital Linear Tape (DLT) jukebox with a physical capacity of 5 TB, or 10 TB data input using vendor-supplied compression firmware. Twelve-inch WORM optical disks cost about five cents/MB, whereas the DLT media cost will only be about 0.4 cents/MB. This media price advantage, the rewritable nature of the tapes, and the increasingly widespread use of DLT make DLT the medium of choice for expansion of NDADS's mass storage capacity at this time. NDADS remains primarily VMS-based, with a small admixture of UNIX.
The newly heterogeneous NDADS will work as follows: NSSDC-developed core software modules will receive data selection specifications from any of the available user interfaces (identified below), consulting a single Sybase inventory data base to determine which jukebox has the needed data, along with which volume(s) and file(s), and calling the software "fetcher" appropriate to that jukebox to actually retrieve the right volume from its slot. It then loads it to a drive and accesses the desired data. Typically, data are then staged to magnetic disk for ANON/FTP retrieval by users. Options exist to have NDADS send data to a user's machine, but this has the disadvantage of requiring the electronic transmission of passwords.
Higher-level interfaces available to users for access to NDADS data include the original E-mail based Automated Retrieval Mail System (ARMS) interface, a World Wide Web (WWW)-based ARMS interface, and the more recent and WWW-accessible Web Interface for Searching Archival Research Data (WISARD) and Space Physics Catalog (SPyCAT) interfaces created for astrophysics and space physics data retrieval, respectively.
NSSDC welcomes reader feedback on the performance and functionality of NDADS and its interfaces.
Erin D. Gardner, email@example.com, (301) 286-0163
Hughes STX, Code 633, NASA Goddard Space Flight Center
Greenbelt, MD 20771, U.S.A.