It isn't, and the problem is not in "hashing files" (or blocks for that matter). Checksum calculation is very cheap, especially if you use default settings (whi

It isn't, and the problem is not in "hashing files" (or blocks for that matter). Checksum calculation is very cheap, especially if you use default settings (which prefer simple & fast checksum functions like crc32/xxhash instead of strong cryptographic functions like sha256/blake2 that can be enabled manually). The real issue is that modifying large files on a COW filesystem causes it to spread that file's guts all over the disk (i.e. fragmentation), and results in serious performance loss over time. For some reason that doesn't happen nearly as much on ZFS and it's totally fine for VM and database workloads.