Thanks for the info and suggestions.
I did a few calculations to check whether a 4GB file size seems appropriate. My six variables are $32, $15, 8, $1, 8, 8 in length for a total of 72 bytes per row. 56x10e6 rows x 72 bytes per row / 1.07x10e9 bytes per GB is about equal to 3.8GB. The character formats are as small as they can be, but you're right that I could reduce two of these to just 8 bytes if I use numeric values with a format.
If I replace two of the character values with numeric values (and create a format for each of the unique character strings), my six variables would still come to 41 bytes per row which calculates to about 2.1GB. This is too close to the limit for my comfort.
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