How is data written to a pool with multiple VDevs

[Danny Carroll]

 Hello All,

I'm in the process of upgrading my home file server and at the same time
I'm going to reorganise the pool.
Currently it looks like this:

nas# zpool status
  pool: areca
 state: ONLINE
 scrub: scrub completed after 12h59m with 0 errors on Sun Aug 29
16:09:09 2010
config:

        NAME        STATE     READ WRITE CKSUM
        areca       ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            da0     ONLINE       0     0     0
            da1     ONLINE       0     0     0
            da2     ONLINE       0     0     0
            da3     ONLINE       0     0     0
            da4     ONLINE       0     0     0
            da5     ONLINE       0     0     0
            da6     ONLINE       0     0     0
            da7     ONLINE       0     0     0
            da8     ONLINE       0     0     0
            da9     ONLINE       0     0     0
            da10    ONLINE       0     0     0
            da11    ONLINE       0     0     0


When I am done I will be using larger drives and I want it to look like
this (I did not know about the 6 drive per vdev recommendation when I
created the array originally):

nas# zpool status
  pool: areca
 state: ONLINE
 scrub: scrub completed after 12h59m with 0 errors on Sun Aug 29
16:09:09 2010
config:

        NAME        STATE     READ WRITE CKSUM
        areca       ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            da0     ONLINE       0     0     0
            da1     ONLINE       0     0     0
            da2     ONLINE       0     0     0
            da3     ONLINE       0     0     0
            da4     ONLINE       0     0     0
            da5     ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            da6     ONLINE       0     0     0
            da7     ONLINE       0     0     0
            da8     ONLINE       0     0     0
            da9     ONLINE       0     0     0
            da10    ONLINE       0     0     0
            da11    ONLINE       0     0     0


I plan to do the migration in 2 steps.
1. Put 6 of the new drives into a spare machine I have and create a 6
disk raidz array.
2. Copy the data accross.
3. Import those 6 drives into the old server and add the remaining 6 to
the pool in a second raidz vdev.

I believe that data is striped accross the 2 vdevs much like a raid0
array stripes across 2 drives.

My question is:  Since the data will initially be on only one of the
raidz vdevs, when I add the second one, will ZFS re-organise the data on
the whole pool to be efficient or will I end up with only new data being
striped.
If that is the case then I will try and find a way to build the new
array fully on a temporary machine first before copying the data.

-D