Aspects include testing distributed file systems by selecting a file in a multiple writer environment and selecting an offset of a block in the file. Test data is generated for the block by randomly selecting a starting value from a plurality of possible starting values. A test header that includes the starting value and a test data sequence that starts with the starting value is created. A file system that is being tested writes the test header and the test data sequence to the block. Contents of the block are read by the file system that is being tested, and expected contents of the data sequence are determined based on contents of the read header. The expected contents of the data sequence are compared to the read data sequence and an error indication is output based on the expected contents not being equal to the read contents.