It appears that some users have complained that they have to delete some music and photo files in order to update to the new ios8 as it takes up to 5.8 gb of storage space. It is worth reading this article before you do so - especially the last couple of paragraphs.
http://www.bbc.co.uk/news/technology-29251961
http://www.bbc.co.uk/news/technology-29251961