What does "scan[ning] all shared folders" actually DO?

[Reliken]

I gather that I need to do that if I want my .mp3s to work, and it allows the generation of thumbnails, but...

What is actually going on when I scan all shared folders? What is my computer doing? If it's generating/saving files, where is it saving them to? How much space could the generation of these files feasibly eat up?

I've tried searching on the forums and elsewhere on the Internet but haven't really been able to find this question - sorry if it's rather simple.

[Reliken]

Well... Not only did "scanning all shared folders" not make my .mp3s work (they all still show up as unplayable files) and not generate thumbnails, but since doing it PS3 Media Server is showing a new error in the Traces.log whenever I start the server:

INFO 2012-10-25 23:30:28.054 [New I/O server worker #11-4] Node(net.pms.dlna.virtual.MediaLibrary) already has an ID=0$9, which is overriden now. The previous parent node was:RootFolder[[MapFile [name=HD Films, id=0$1, format=null, children=[]], MapFile [name=Television, id=0$2, format=null, children=[]], MapFile [name=SD Films, id=0$3, format=null, children=[]], MapFile [name=Batman Cartoons, id=0$4, format=null, children=[]], MapFile [name=Dragon Ball Z, id=0$5, format=null, children=[]], MapFile [name=Christmas Specials, id=0$6, format=null, children=[]], MapFile [name=Music, id=0$7, format=null, children=[]], VirtualFolder [id=8, name=Web, full path=0$8, ext=null, discovered=false]]]

... what the heck? This is for streaming to an Xbox 360, by the way.

[taconaut]

What is actually going on when I scan all shared folders? What is my computer doing? If it's generating/saving files, where is it saving them to? How much space could the generation of these files feasibly eat up?

Lets start from the beginning When playing a file, pms needs some information like codec, length etc. If the media library is disabled, this information is being read from the file each time it is being played. If the media library is enabled, but no files have ever been played and you haven't manually scanned the shared folders, pms will read this information for each file contained in a folder when opening it on the ps3; it then stores this information in the database. This way it doesn't have to re-read from file the next time, as it takes it from the database (which is quicker). Finally to your question: When manually triggering a scan, all shared folders and sub-folders will be scanned and the file informations will be stored in the database (the same way as if you browse a folder on the ps3).

Using the media library won't influence the way a file is being played back at all, it will only make the folder and file loading quicker, when browsing a folder on the ps3.