Unable to create ZVOL error since upgrading to 9.3-RELEASE

[Paul Mather]

I have a nightly backup job that creates a snapshot of my ZFS pool, 
mounts the snapshot file sets appropriately under /backup, and then 
rsyncs that hierarchy to another system.  This worked fine under 
9.2-RELEASE and before.

Since upgrading to 9.3-RELEASE, I get the following error in the daily 
security run output for that machine:

=====
redmine kernel log messages:
+++ /tmp/security.6itlEbCD	2014-07-30 03:01:10.401183488 -0400
+ZFS WARNING: Unable to create ZVOL sys/SWAP/swap at backup (error=17).
=====

I have swap on a ZFS ZVOL: sys/SWAP/swap.  I create the snapshot via 
"zfs snapshot -r sys at backup" ("sys" is the name of my ZFS pool).

Does anyone know why I should be getting this error now?  The same 
script running on the same machine gave no problems under 9.2-RELEASE.  
I started getting this message as soon as I upgraded to 9.3-RELEASE.

Did some sysctl default change between 9.2-RELEASE and 9.3-RELEASE that 
changed the behaviour of ZVOLs and snapshots?

The message isn't fatal, but it is annoying to have it appear every day 
in the security run output.

Cheers,

Paul.