"Modern" file systems (say, ext3 and onwards) scales pretty well. The problem is that most tools that operate on file systems doesn't. Things like readdir() is

"Modern" file systems (say, ext3 and onwards) scales pretty well. The problem is that most tools that operate on file systems doesn't. Things like readdir() is a linear scan and takes a long time on a million files. So in practice it's not optimal. A thousand files, no problem. A million, start looking at doing it in-memory (or use some database tool).