[NOOB QUESTION]Need information about "repo sync".

Hello ! I'm a newcomer in the Android scene, (so I'm a n00b), and i want to know informations about the AOSP. I'm currently synchronizing a repo on my computer (ubuntu 14.04 LTS), i have learned how to do it from tutorial from the official AOSP site. But here is my question : at 10:00 (this morning) , I've typed in the console "repo sync". And from 10:00, terminal downloading files. Is it normal it takes so long to download android source files ? Thanks for any answers.

PS : "working_directory" folder is 20GB+ at least...

Also, sorry for my bad English.

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