Test format was found hung on zSeries after 11 hours.
Attempting x86 repro now:
CONFIG
############################################ # RUN PARAMETERS ############################################ abort=0 auto_throttle=0 backups=0 bitcnt=4 bloom=1 bloom_bit_count=52 bloom_hash_count=15 bloom_oldest=1 cache=3 checkpoints=1 checksum=uncompressed chunk_size=1 compaction=0 compression=lz4 data_extend=0 data_source=table delete_pct=6 dictionary=0 direct_io=0 encryption=none evict_max=4 file_type=row-store firstfit=0 huffman_key=0 huffman_value=0 in_memory=0 insert_pct=25 internal_key_truncation=1 internal_page_max=10 isolation=random key_gap=5 key_max=92 key_min=15 leaf_page_max=17 leak_memory=0 logging=0 logging_archive=0 logging_compression=none logging_prealloc=0 long_running_txn=0 lsm_worker_threads=3 merge_max=7 mmap=1 ops=100000 prefix_compression=0 prefix_compression_min=3 quiet=1 repeat_data_pct=89 reverse=0 rows=100000 runs=100 rebalance=1 salvage=1 split_pct=76 statistics=0 statistics_server=0 threads=3 timer=20 transaction-frequency=24 value_max=656 value_min=13 verify=1 wiredtiger_config= write_pct=68 ############################################