When i try to offline a disk in a zfs raidz pool (the raidz pool is not mirrored), zfs says that the disk cannot be taken offline because it has no valid mirror.
Isn't one of the properties of raidz that it has a redundant disk (or even 2 disks in raidz2)...?
Could you give a bit more about your configuration please? What are the commands you are using? If I'm understanding your question that should work.
Note that:
You cannot take a pool offline to the point where it becomes faulted. For example, you cannot take offline two devices out of a RAID-Z configuration, nor can you take offline a top-level virtual device.
Managing Devices in ZFS Storage Pools: http://docs.huihoo.com/opensolaris/solaris-zfs-administration-guide/html/ch04s04.html
ZFS Best Practices Guide: http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide