Processing Satellite Images on Tertiary Storage: A Study of the Impact of Tile Size on Performance

In July 1997, NASA will begin to launch a series of 10 satellites as part of its Mission to Planet Earth, more popularly known as EOSDIS (for Earth Observing System, Data Information System). When fully deployed, these satellites will have an aggregate data rate of about 2 megabytes a second. While this rate is, in itself, not that impressive, it adds up to a couple of terabytes a day and 10 petabytes over the 10 year lifetime of the satellites [1]. Given today’s mass storage technology, the data almost certainly will be stored on tape. The latest tape technology offers media that is very dense and reliable, as well as drives with transfer rates in the same range as magnetic disk drives. For example, Quantum’s DLT4000 drive has a transfer rate of about 3.0 MB/sec (compressed). The cartridges for this drive have a capacity of 40 GB (compressed), a shelf life of 10 years, and are rated for 500,000 passes [2]. However, since tertiary storage systems are much better suited for sequential access, their use as the primary medium for database storage is limited. Efficiently processing data on tape presents a number of challenges [3]. While the cost/capacity gap [4] between tapes and disks has narrowed, there is still about a factor of 2 in density between the best commodity tape technology (20 gigabytes uncompressed) and the best commodity disk technology (10 gigabytes uncompressed) and at least a factor of 4 in total cost ($2,000 for a 10 GB disk and $10,000 for a 200 GB tape library).