Storing files in directories in an online file manager

I have a site that allows people to upload files to 开发者_JAVA百科their account, and they're displayed in a list. Files for all users are stored on different servers, and they move around based on how popular they are (its a file hosting site).

I want to add the ability for users to group files into folders. I could go the conventional route and create physical folders on the hard drive, for each user on the server, and transverse them as expected. The downside to that is the user's files will be bound to a single server. If that server starts running of space (or many files get popular at the same time), it will get very tricky to mitigate it.

What I thought about doing is keeping the stateless nature of files, allowing them to be stored on any of the file servers, and simply storing the folder ID (in addition to the user id who owns the file) with each file in the database. So when a user decides to move a file, it doesn't get physically moved anywhere, you just change the folder ID in the database.

Is that a good idea? I use php and mysql.

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Yes, it is.

I don't see any downside, except maybe more queries to the database, but with proper indexing of the parent folder id, this will probably be faster than accessing the filesystem directly.

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Forget about folders and let the users tag their files, multiple tags per file. Then let them view files tagged X. This isn't much different to implement than virtual folders but is much more flexible for the users.