What are File and Directory ID's?
On a mac the file system stores all files and directory information on each volume in a big file called the catalogue file. Inside the catalogue, all files and directories are accessed using a key, central to which is a number, called the ID. In the case of a file its a file ID (FID) or for a directory, a directory ID (DID).
How many IDs can there be?
The ID in a catalogue key is stored using 4 bytes, which means it can only be between 0 and 4,294,967,295 (FF FF FF FF in hex). However the first 16 IDs are reserved so you don't have quite that many. Each ID is unique for ever, on any given volume. Once all 4 billion have been used, you cannot create new files, so will need to reformat the volume to continue using it.
Why are IDs so important?
Most system calls relating to files inside a mac (either vi a network or a hard disk) can refer to files or directories by ID. This makes the ID a powerful piece of information.
So whats the problem?
The problem lies in file servers that don't use IDs. The protocol used by macs to share files over a network is called the Apple Filing Protocol (AFP) and it requires the use of IDs. So if you want to support AFP fully, any AFP server, must adopt its own system for storing and maintaining a link between each file or directory and its respective ID. This is most critical when acessing directories.
So why does this matter on a non mac server like netatalk?
The three big stumbling blocks that crop up with AFP servers that don't fully support IDs are 1) aliases, 2) the trash can and 3) linked documents.
An alias on a mac is quite special. Rather than just storing the path to the original file (like a unix symlink), it stores the ID to that file and a special identifier for the volume (and the server it's on). Ideally this is great. If the file moves or is renamed, the alias still works. However if either the file (or directory) ID changes, or the volume identifier (or server identifer), then the alias will break. The file it claims to point to will claim to have been removed.
Trash can (accidentally deleted file) problems
The trash can has similar problems. Files that have been moved to the trash are represented by their ID. When you empty the trash all ID's listed are deleted. However if the ID of a file that was in the trash, is reallocated to an ordinary file, then when the trash is emptied that file will be deleted.
Linked document problems
Finally linked documents: Linked documents are documents that contain hidden links to other documents. Print setting and layout application (such as Quark) use this technique. Sometimes these documents contain IDs linking to their embeded documents. These can break in the same way as aliases.