ZFS does support different sized drives, and you can add disks to a pool easily.

It does not support different sized drives in a raidz (equivalent to an mdadm array), as far as I know, but you can mix raidz and singular drives in a pool, though that is a bit silly. It also does not support adding disks to a raidz, which is equivalent to growing a raid5 or raid6 array, which is admittedly a pain in the ass.

You can also upgrade an entire raidz vdev if you replace each drive in the vdev with a larger one (one at a time) and enable "autoexpand" on the zpool. It's a manual and time consuming process, waiting for each drive to resilver. Still an pain, but at least possible.

> It does not support different sized drives in a raidz

  % mkfile -v 64m /var/tmp/d0
  /var/tmp/d0 67108864 bytes
  % mkfile -v 96m /var/tmp/d1
  /var/tmp/d1 100663296 bytes
  % mkfile -v 128m /var/tmp/d2
  /var/tmp/d2 134217728 bytes
  % sudo zpool create -f pool0 raidz /var/tmp/d0 /var/tmp/d1 /var/tmp/d2
  Password:
  % zpool status
    pool: pool0
   state: ONLINE
    scan: none requested
  config:

        NAME             STATE     READ WRITE CKSUM
        pool0            ONLINE       0     0     0
          raidz1-0       ONLINE       0     0     0
            /var/tmp/d0  ONLINE       0     0     0
            /var/tmp/d1  ONLINE       0     0     0
            /var/tmp/d2  ONLINE       0     0     0

  errors: No known data errors

You can build it, but it is limited to the smallest device in the vdev, so the array will appear to only have three devices of 64m available. Though you can replace all smaller disks and grow it.

That is not how RAIDZ works. Because every write is a dynamic width stripe, the capacity is an aggregate minus parity, not linear.

The smallest device capacity applies to RAID1+0, which is mirroring, but it does not apply to RAIDZ.

Of course that ZFS supports any sized drives in a pool. ZFS expands very easily, in fact, the user doesn't have to do anything, it automatically expands.

Of course you can remove drives in ZFS. How else could you replace disks?

I downvoted your post because every statement you made about ZFS is wrong.

You're describing operations that can be done on a pool, while conveniently ignoring that they can not be done to a single RAID array (vdev), which is the more useful comparison against btrfs.

Yes, you can add drives of arbitrary size to a ZFS pool, but when put into the same array, they all get treated as the size of the smallest one. Yes, you can expand a ZFS pool but only by adding a new array or by incrementally upgrading all the drives in the new array. Yes, you can remove drives, but only for replacement purposes; you can't do an in-place rebuild to make the reduced number of drives the new normal instead of degraded mode.

I know nothing about ZFS, so sorry if these are dumb questions.

Can you have an array that contains a single disk? And if you can, then

* Can you add as many single drive arrays to a pool as you like?

* Can you migrate data off of a given single-drive array on to other single-drive arrays in the pool and then permanently remove the array from the pool?

Yes, yes, no. Plus a single disk array has no redundancy.

Minor nit: depending on your definition of redundancy, you may set copies=n, on a single vdev pool. You won't get hardware redundancy but will get data redundancy.