"Increase disk utilization when using Swift"? What you wrote makes no sense.

There is never any fear of losing a pool when losing a vdev, as ZFS will let one simulate the entire thing by letting one use files as disks:

  % mkfile -v 128m /var/tmp/d0
  /var/tmp/d0 134217728 bytes
  % mkfile -v 128m /var/tmp/d1
  /var/tmp/d1 134217728 bytes
  % sudo zpool create testpool0 mirror /var/tmp/d0 /var/tmp/d1
  Password:
  % zpool status
  pool: testpool0
  state: ONLINE
  scan: none requested
  config:

	NAME             STATE     READ WRITE CKSUM
	testpool0        ONLINE       0     0     0
	  mirror-0       ONLINE       0     0     0
	    /var/tmp/d0  ONLINE       0     0     0
	    /var/tmp/d1  ONLINE       0     0     0

  errors: No known data errors
  % zfs list
  NAME        USED  AVAIL  REFER  MOUNTPOINT
  testpool0   988K  79.0M   952K  /Volumes/testpool0
  % sudo zpool set autoexpand=on testpool0
  % mkfile -v 192m /var/tmp/d2 
  /var/tmp/d2 201326592 bytes
  % mkfile -v 192m /var/tmp/d3 
  /var/tmp/d3 201326592 bytes
  % sudo zpool replace testpool0 /var/tmp/d0 /var/tmp/d2
  % sudo zpool replace testpool0 /var/tmp/d1 /var/tmp/d3
  % zfs list
  NAME        USED  AVAIL  REFER  MOUNTPOINT
  testpool0  1008K   143M   952K  /Volumes/testpool0
  % sudo zpool destroy testpool0
  Running process: '/usr/sbin/diskutil' 'unmount' '/Volumes/testpool0' 
  Unmount successful for /Volumes/testpool0
  % rm /var/tmp/d[0-9]