ZFS stalled after some mirror disks were lost

[Ben RUBSON]

Hi,

On a FreeBSD 11 server, the following online/healthy zpool :

home
  mirror-0
    label/local1
    label/local2
    label/iscsi1
    label/iscsi2
  mirror-1
    label/local3
    label/local4
    label/iscsi3
    label/iscsi4
cache
  label/local5
  label/local6

A sustained read throughput of 180 MB/s, 45 MB/s on each iscsi disk
according to "zpool iostat", nothing on local disks (strange but I
noticed that IOs always prefer iscsi disks to local disks).
No write IOs.

Let's disconnect all iSCSI disks :
iscsictl -Ra

Expected behavior :
IO activity flawlessly continue on local disks.

What happened :
All IOs stalled, server only answers to IOs are made to its zroot pool.
All commands related to the iSCSI disks (iscsictl), or to ZFS (zfs/zpool),
don't return.

Questions :
Why this behavior ?
How to know what happens ? (/var/log/messages says almost nothing)

I already disconnected the iSCSI disks without any issue in the past,
several times, but there were almost no IOs running.

Thank you for your help !

Ben