zfs discuss - Mar 2010 - checksum errors increasing on "spare" vdev?

Hi,
One of my colleagues was confused by the output of ''zpool
status'' on a pool
where a hot spare is being resilvered in after a drive failure:

$ zpool status data
  pool: data
 state: DEGRADED
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
 scrub: resilver in progress for 0h56m, 23.78% done, 3h1m to go
config:

        NAME          STATE     READ WRITE CKSUM
        data          DEGRADED     0     0     0
          raidz1      ONLINE       0     0     0
            c0t2d0    ONLINE       0     0     0
            c1t2d0    ONLINE       0     0     0
            c0t4d0    ONLINE       0     0     0
            c0t5d0    ONLINE       0     0     0
            c1t4d0    ONLINE       0     0     0
            c0t7d0    ONLINE       0     0     0
          raidz1      DEGRADED     0     0     0
            spare     DEGRADED     0     0 2.89M
              c0t1d0  REMOVED      0     0     0
              c0t6d0  ONLINE       0     0     0  59.3G resilvered
            c1t5d0    ONLINE       0     0     0
            c0t3d0    ONLINE       0     0     0
            c1t1d0    ONLINE       0     0     0
            c1t3d0    ONLINE       0     0     0
            c1t6d0    ONLINE       0     0     0
        spares
          c0t6d0      INUSE     currently in use

The CKSUM error count is increasing so he thought that the spare was also
failing.  I disagreed because the errors were being recorded on the
"fake" vdev
"spare", but I want to make sure my hunch is correct.

My hunch is that since reads from userland continue to come to the pool, and
since it''s raidz, some of those reads will be for zobject addresses on
the
failed drive, now represented by the spare.  Because the data at those addresses
is uninitialized, we get checksum errors.

I guess I really have two questions:
1. Am I correct about the source of the checksum errors attributed to the
"spare" vdev?
2. During raidz resilver, if a read happens for an address that is among
what''s
already been resilvered, will that read succeed, or will ALL reads to that
top-level vdev require reconstruction from the other leaf vdevs?

If the answer to #2 is that reads will succeed if they ask for data
that''s been
resilvered, then I might expect my read performance to increase as resilver
progresses, as less and less data requires reconstruction.  I haven''t
measured
this in a controlled environment though, so I''m mostly just curious
about the
theory.

Eric