We analyze the manycore scalability of five widelydeployed file systems, namely, ext4, XFS, btrfs, F2FS, and tmpfs, by using our open source benchmark suite, FXMARK. FXMARK implements 19 microbenchmarks to stress specific components of each file system and includes three application benchmarks to measure the macroscopic scalability behavior. We observe that file systems are hidden scalability bottlenecks in many I/Ointensive applications even when there is no apparent contention at the application level. We found 25 scalability bottlenecks in file systems, many of which are unexpected or counterintuitive. We draw a set of observations on file system scalability behavior and unveil several core aspects of file system design that systems researchers must address.