[jackdied]

Definitely, you don't want to scan the filesystem to do searches. But finding user 1234's profile in profiles/1234.data is easy. In my experience this works well (even with the wasted space) though operations like fsck start to get painful when you have 1 million+ files.

An anecdote on bad linear search: in the 90s I worked on a system in C++ that made heavy use of inheritance and other C-plus-plus-ery. There was a bunch of file opener classes each that found files in a different way (regexp, straight path, glob). They differed only in a method named match. The base class called match on every file name in the directory until it returned true. That was fine for regexps but even if you opened a file _by name_ it took N/2 syscalls to readdir to find it! This worked OK until someone created a directory with 10k entries on a production box. Ouch.