Apple Fusion Drive
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.
Boot into recovery
The iMacs that come with Fusion also come with a Rescue partition that runs itself from RAM (a virtual disk2). This will allow you to play with the hdd and sdd without problems caused by mounted disks. Power on your iMac, holding down apple+R to enter the recovery environment.
When the Recovery has started, start a Terminal (under menu 'Utilities).
Destroy CoreStorage and prepare new volumes
Find the CoreStorage 'Logical volume group' ID, and take note of the ssd and larger disk (disk0 & 1)
diskutil cs list
Remove the CoreStorage volume
diskutil cs delete <VOLUMEID>
Sometimes you need to unmount a volume or try the above command twice for OSX to 'understand' and actually run it successful.
Zero the ssd (if you specify the correct disk this takes ~ 5 minutes) This will remove everything including the partition table so OSX doesn't 'see' the disk and won't try to use it during installation.
diskutil zeroDisk disk1
Erase the hdd (this is faster then zeroDisk plus it creates a new HFS+ volume for OSx)
diskutil eraseDisk JHFS+ Macintosh disk0
Install OSX on hdd
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.
Proceed installing Archlinux
Installing Archlinux requires no special tricks; follow the Installation Guide; mount /dev/sda1 (the first partition of the hdd is the UEFI partition) on /boot/efi; install grub-efi-x86_64 and you should be good to go! Since you will be installing on the Solid_State_Drives, use parted and created aligned partitions. I use this scheme:
Model: ATA APPLE SSD SM128E (scsi) Disk /dev/sdb: 121332826112B Sector size (logical/physical): 512B/4096B Partition Table: gpt Disk Flags: Number Start End Size File system Name Flags 1 1048576B 16383999999B 16382951424B linux-swap(v1) 2 16384000000B 121331777535B 104947777536B ext4
Also, to use the internal network-card you will need at least kernel 3.9! I use a thunderbolt cable (which works in kernel 3.7+) to download the latest 3.9-rc