With the development of high-performance comput ing, 110 issues have become the bottleneck for many massively parallel applications. This paper investigates scalable parallel 110 alternatives for massively parallel partitioned solver systems.Ty pically such systems have synchronized "loops" and will write data in a well defined block 110 format consisting of a header and data portion. Our target use for such an parallel 110 subsystem is checkpoint-restart where writing is by far the most common operation and reading typically only happens during either initialization or during a restart operation because of a system failure. We compare four parallel 110 strategies: 1 POSIX File Per Processor (lPFPP), a synchronized parallel 10 library (synclO), "Poor-Man's" Parallel 110 (PMPIO) and a new "reduced blocking" strategy (rbIO). Performance tests using real CFD solver data from PHASTA (an unstructured grid finite element Navier-Stokes solver [1]) show that the synclO strategy can achieve a read bandwidth of 6.6GB/Sec on Blue Gene/L using 16K processors which is significantly faster than 1PFPP or PMPIO approaches. The serial "token-passing" approach of PMPIO yields a 900MB/sec write bandwidth on 16K processors using 1024 files and 1PFPP achieves 600 MB/sec on 8K processors while the "reduced-blocked" rblO strategy achieves an actual writing performance of 2.3GB/sec and perceivedliatency hiding writing performance of more than 21,000 GB/sec (i.e., 21TB/sec) on a 32,768 processor Blue Gene/L.