Writes in the POSIX API can be atomic depending on the underlying filesystem. For example, small writes on ZFS through the POSIX API are atomic since they eithe

Writes in the POSIX API can be atomic depending on the underlying filesystem. For example, small writes on ZFS through the POSIX API are atomic since they either happen in their entirety or they do not (during power failure), although if the writes are big enough (spanning many records), they are split into separate transactions and partial writes are then possible: https://github.com/openzfs/zfs/blob/34205715e1544d343f9a64149b17c371282306ae/module/zfs/zfs_vnops.c#L755 https://github.com/openzfs/zfs/blob/34205715e1544d343f9a6414... Writes on ZFS cease to be atomic around approximately 32MB in size if I read the code correctly.