While WTX is available in 64bit, its C-API (runmerc.h) remained 32bit. This leads to a bottleneck. WTX mappings without C-API EXIT() calls work with huge files (>4GB), but WTX mappings that make use of the C-API EXIT() call can handle 1.9GB only.
Please also consider to migrate JEXIT(), the Java API, to 64bit.
P.S.: This FITS was already submitted under a differents brand/product while B2B/WTX was not available on the RFE platform, see http://www.ibm.com/developerworks/rfe/execute?use_case=viewRfe&CR_ID=37365
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