Apple recently added two new features to iTunes: “iTunes in the Cloud”, which adds all your iTunes music purchases to a cloud library which can be streamed or downloaded as often as you want to any of your devices (iPhone, iPod, iPad, computer with iTunes), and “iTunes Match”, which adds all the music you didn’t buy from iTunes to that same cloud library, enabling all the same “iTunes in the Cloud” features.

Apple is billing this primarily as a way to get your music from your iTunes library onto your phone or iPad without having to plan ahead and pre-sync the music onto the device.

However, in addition to using iCloud to sync music between devices, iTunes Match also has another benefit: it syncs the library metadata between multiple iTunes libraries that enable iTunes Match using the same account.

This subtle addition is actually something important that I’ve wanted from iTunes as long as I’ve been using it: iTunes has a bunch of organization features including smart playlists which work best if it knows what you listen to and how much you like it. It tracks how many times you play each song and allows you to rate songs, and it automatically syncs this between one iTunes library and any connected iPods, but until now each iTunes library has been an island: if you have two computers, say a desktop and a laptop, each has its own library, with its own play counts and ratings and playlists, and its own idea of your preferences, and any iPods or other Apple devices you have were able to sync songs and metadata from a single one of these libraries.

Now, with iTunes Match enabled, all my computers with iTunes see the same library — all of the songs on any one of them are synced up to iCloud and then back to the other computers, and metadata including play counts, ratings and playlists are also kept in sync. This joining of the libraries also means it matters a lot less which of my computers I sync a given iPod with, since effectively they all use the same library.

There’s one minor remaining annoyance given the way I use iTunes at home: I have a file server with all my music, a Mac Mini in the living room which can play music and movies, and my main desktop computer, and each of these Macs uses the network file server for the music files instead of storing a local copy. (My laptop spends enough time disconnected from the home network that it does deserve a local copy of the music.) This means that if I add music to the library on one of the desktop computers, it’s already accessible in the expected location from any of the other computers that can access the file server, though the other computers won’t know it until I tell iTunes to add the files to the local library. The storage is shared; the library metadata is still per-computer. Using iTunes Match, the libraries are kept in sync automatically, but iTunes doesn’t notice that some of the files are magically already local — it will show such files with the cloud icon, and offer to stream them or download them even though that’s unnecessary. Streaming works fine, and downloading works, but actually downloads the file to the same folder, notices that the file already exists, and adds a numeric suffix to make the filename unique.

It would be nice if there was a way to tell iTunes to notice local copies of files that it thinks are only in iCloud. Also, I occasionally found myself having downloaded extra copies of songs, noticing this cluttering up the music folder, and deleting the extra files outside of iTunes (using Finder) — this left me in a broken state where iTunes has an entry for the song in the library, and the file exists in the filesystem and in iCloud, but iTunes thinks it’s local so won’t stream or download from iCloud and thinks it has a different name so it won’t play the local file, at which point things are pretty broken. What’s needed here is a way to stuff the file back into the cloud, making iTunes think there is no local copy so it will go get it from iCloud again the next time it’s needed — I couldn’t figure out how to do this until I read this Q&A article at Ars Technica which has the secret: select the song, hold the option key while pressing the delete key, and the normal delete confirmation dialog will grow a new checkbox “also delete this song from iCloud”. Leaving that option unchecked, you can delete the local copy of the song (or, a broken link to a missing local copy of a song) while leaving it in the cloud library, the cloud icon comes back, and iTunes will go back to getting the data from iCloud as needed again.