A couple of months ago I began receiving constant e-mail alerts stating that my FreeNas box was 80% full. My 2-year-old setup had 4 2TB Seagate drives in a Raidz1 pool. After some research and test with new firmware builds I found out that this was not optimal since the Raidz1 should follow the 2*n+1 formula [with n>0] (3, 5, 7 or 9 … drives).
I cannot afford to rebuild the pool at this time and one of the original drives failed and was replaced by a newer 4TB unit.
My approach was to replace every remaining 2TB drive on the pool by a 4TB one and this proved to be very time consuming. My box was taking too long to resilver the pool.
After some more research I came across Allan Jude’s “ZFS Advanced Topics” chapter proposed to the FreeBSD documentation project.
sudo sysctl vfs.zfs.resilver_delay=0 sudo sysctl vfs.zfs.scrub_delay=0
These tunables reduce the wait time between each resilver and scrub IO operation. Client performance was somewhat degraded but getting my pool back into pristine condition was more important.