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I have an iPhone 3GS 32GB which was packed to the brim with music. Yesterday I signed up for iTunes Match, and I noticed that the Music section of the device settings in iTunes (which used to allow me to select which playlists/songs/genres/artists I wanted synced) has been replaced with this:

iTunes Screenshot

iTunes also says my phone has 22.6 GB of free space, despite most of the bar appearing full.

iTunes screenshot

How does my iPhone manage its free space when the device fills up? Will downloaded music automatically be removed when something else needs it, like an app installation or a new podcast? If so, how does it decide what to remove? Or, will I have to manually remove songs one by one if I decide to install all 782 MB of Rage HD again?

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Interestingly, the Free space is exactly one GB less than the Audio space. Possibly a coincidence, but could suggest that iTunes reserves 1GB of space for music. –  Nathan Greenstein Jan 10 '12 at 5:15
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It's annoying that this still hasn't been answered satisfactorily :( –  Django Reinhardt Aug 2 '12 at 23:13
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4 Answers 4

Some research led to this article from AllThingsD about how an Apple rep had confirmed that the service does indeed stream the music, but it also saves to the device, thus taking up storage space. It starts playing before it finishes downloading, but it does actually download and save.

MacRumors and a video from InsanelyGreatMac are somewhat unclear about this, but MacWorld agrees that on an iOS device, it's downloading (scroll down to "Playing iTunes Match songs on your Mac and on iOS").

UPDATE as per comment: As of iOS 5, the device will delete cached data from apps when it's low on space. This caused some controversy at first because apps had no way to prevent cached data from being removed automatically. In the linked article, Marco Arment provides an example:

A common scenario: an Instapaper customer is stocking up an iPad for a long flight. She syncs a bunch of movies and podcasts, downloads some magazines, and buys a few new games, leaving very little free space. Right before boarding, she remembers to download the newest issue of The Economist. (I think highly of my customers.) This causes free space to fall below the threshold that triggers the cleaner, which — in the background, unbeknownst to her — deletes everything that was saved in Instapaper. Later in the flight, with no internet connectivity, she goes to launch Instapaper and finds it completely empty.

This was fixed in iOS 5.0.1 (Apple Developer documentation) so that developers can differentiate between cached data that is and isn't important.

So, the short version: Once free space gets below a certain level, the device will regain whatever space it can by removing cached data from installed apps. If it can't do that, or if you're completely out of space, it'll probably just tell you that you don't have enough space to do what you're trying to do.

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I'm aware that the songs get saved to the device as I play them, but my question relates to what happens when the device fills up. –  Brant Bobby Jan 8 '12 at 20:31
    
@BrantBobby: updated answer as per your comment. I'm not sure it gets at the root of your question, though. –  Mr. Jefferson Jan 11 '12 at 17:56
    
This doesn't answer the original question at all! :( –  Django Reinhardt Aug 2 '12 at 23:13
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Last week iTunes Match deleted almost every song that I have on my iPhone leaving just 10 songs on the device. The reason may be that while I updated several large Apps, it had not enough space to store the updates and the original App, so it cleaned up the memory which also includes the iPod Library :/

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From what I've heard from some developers, it deletes the oldest least played music first. This obviously makes sense. And if your apps need more space they get priority over music. But it's not one of those things I worry about. My iPhone has plenty of space for apps and if music gets deleted the playcount and ratings are preserved in the cloud so I don't worry about it.

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If you want to delete all the music from your iPhone to free up space: Settings>General>Usage and wait for Music to show up under Storage. Click on Music and then Edit from the top right, click the red slash on the left and Delete shows up. Click to delete and all your music files will be deleted from your iPhone.

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