Hi Craig,
somebody in the thread mentioned that they get abysmal performance with zvols (chunks of a zpool allocated to be a "disk" - similar to an LVM logical volume). That really surprises me, I've had fantastic performance from them. All of my VMs are now on zvols....and i've done a lot of testing of zfs in a VM with zpools made up of lots of 100-200MB zvols.
Recently I had a performance problem as well, under FreeBSD. Overnight I filled a 900GB zvol. A backup took too much (the script wasn't working as expected) so at one stage there were just 4GB left. I had our Zimbra mail server running on this machine, inside VirtualBox. That one complained about SCSI timeouts (virtal, inside the VM), and finally remounted the virtal disk read-only (practically "killing" the mail server). The zvol was never running out of disk space, just nearly 100% full. According to other discussions ZFS slows down if it is filling up. That would explain my problem. BTW: zfs receive seems to need additional disk space while "unpacking". It releases some when finished. I did not get my head around to look into it in more detail but should do it, to avoid more "surprises" of this kind. Regards Peter