ZFS has built-in support for redundant disks (in fact, using a lower-level hardware or software RAID under ZFS is not recommended). This allows it to do fancy things like allocate files intelligently across stripes, rebuild a disk with only the data used by the file system (huge time saver), and be able to repair files on-the-fly that are damaged, or at worst, report what files cannot be repaired.
What gave you that impression? With `zpool attach` you can attach a device to an existing VDEV (single device, mirror, RAIDZ) to increase redundancy. With `zpool add` you can add a new VDEV to your pool to extend its capacity.
What he means is you can't add another disk in vdev. If you have a raidz with 3 disks, you can't add 4th one and rebalance. What you can do is either add another vdev to the pool, making it two+ striped raidz or you can replace your 3 disks with a bigger ones.
Reason for that is that adding a disk to vdev will require rebalancing everything and considering complexity of zfs, you better just make a new raidz with 4 disks and move data over, otherwise it might just take ages and complicate things a lot. This is obviously not ideal for a home user, but zfs was not created with a home users in mind.
Btrfs supposedly should let you do this, but their raid5/6 is still unstable - probably for a reason.
In fact, this is not such a big deal. You can add disks to a zfs mirror (and it is real n-way mirror, not btrfs raid1 thing with multiple disks), you can add mirror vdevs to raid10 setup. If you are making raidz1/2/3 just make sure you understand that you can't expand it by adding more disks.
It doesn't. GFS2 is a clustered filesystem, ZFS is not. GFS2 is for sharing a single filesystem between multiple nodes, with a distributed lock manager to avoid corruption. It's complex to fix when it breaks, generally fairly slow and very few people need it - for most users, NFS (especially v4 and up) is a better way of accomplishing the same end goal.
