Best Performance for Max Files in a Directory on Linux File System

If you are using ext3, I found this quote (warning: spanish speaking site)

"There is a limit of 32k (32768) subdirectories in a single directory, a limitation likely of only academic interest, as many people don't even have that many files (though huge mail servers may need to keep that in mind). The ext2 inode specification allows for over 100 trillion files to reside in a single directory"

Further reading showed that ext3 doesn't have a 32K limitation, which can be empirically proven with

a=0; i=1; while [ $a == 0 ]; do touch $i; a=$?; let i++; done

but it does have a 32K folder limit for folder, which can be tested with

a=0; i=1; while [ $a == 0 ]; do mkdir $i; a=$?; let i++; done

This (unfounded) claim says that

ReiserFS has no trouble at all with hundreds of thousands of files in a single directory. flabdablet - February 1, 2007

This question from sister site stackoverflow.com could help too.

In general:

• There is a limit to the amount of directories,
• You should keep your files/directories under 32K, but can go a lot further,
• The file system you are using does matter.

Provided you have a distro that supports the dir_index capability then you can easily have 200,000 files in a single directory. I'd keep it at about 25,000 though, just to be safe. Without dir_index, try to keep it at 5,000.