remove or make reserved ZFS space configurable

[Craig Yoshioka]

I saw an earlier thread about an update that added (invisible) reserved space for ZFS Pools.

I use ZFS as the filesystem for my backup disks.  This allows me to occasionally rotate through old backup drives and scrub them to detect damage without having to implement such detection in my backup system.  I can then re-backup bad files, or reconstruct a backup drive, if necessary.   I configured my backup software to leave ~100GB free (3TB  drives) and I was not amused when I recently mounted a set of backup drives, and puzzled over why they were now "full” and if I had messed up and lost data.  And even with the reserved space, while troubleshooting I tried a rm and it failed (so what's the point of the reserved space?).

I don't care about the performance degradation on these drives... these drives are 99% single-write.  I agree, that having reserved space as a default is probably good, but why isn't this implemented as a configurable option (like a default zpool/zfs quota property)?  Instead it looks like I have to wait for an update to make it to release and then set some kernel option?  To prevent accidental filling of a pool, I usually create a zfs filesystem on each pool with about ~10% reserved space, is this solution not workable?
 
Thanks,
-Craig