Apple Fusion Drive
Installing Archlinux on iMac Fusion (2013) systems
Apple uses a Wikipedia:RAID alternative called CoreStorage to merge the ssd and hdd into a single logical volume (sold as Fusion). We need to remove this volume, so we can reinstall OSx to the hdd without using the ssd.
When booting your iMac, hold down apple+R keys to enter the recovery environment. Make sure you have an internet connection (via ethernet cable or WiFi) for your iMac so it can start 'Internet Recovery' which runs from RAM (a virtual disk2). This will allow you to play with the hdd and sdd without problems caused by mounted disks.
When the Internet Recovery has started, start a Terminal (under menu 'Utilities).
Find the CoreStorage 'Logical volume group' ID, and take note of the ssd and larger disk (disk0 & 1)
diskutil cs list
Unmount the MacintoshHd (might not always be needed) and remove the CoreStorage volume
umount /Volumes/Macintosh\ HD diskutil cs delete <VOLUMEID>
Zero the ssd (if you specify the correct disk this takes ~ 5 minutes)
diskutil zeroDisk disk1
Erase the hdd (this is faster then zeroing plus it creates a new HFS+ volume for OSx)
diskutil eraseDisk JHFS+ Macintosh disk0
Quit the terminal and start the OSX installer. Do not use the GUI based Disk Utility at this point, it will show your disks as errored (red) and will want to fix them for you. This will actually screw things up as it will recreate the CoreStorage volume (even if you choose not to 'fix' anything). The OSX installer should show you 1 disk; which is the created JHFS+ volume on the hdd.
Proceed by installing a fresh OSX version on the hdd. You can play around in OSX, for example use the 'Disk Utility' to resize the OSX partition so you can allocate some space for Linux to use besides the ssd. Now boot the Archlinux usb-stick by holding the left-ALT key while booting your iMac (you might need to use an apple keyboard for this to work). This will show you a bootmenu where you can select the USB drive for booting.
Installing Archlinux requires no special tricks; just follow Installation Guide install grub-uefi and you should be good to go!