Strange ZFS performance

[Wes Morgan]

On Sun, 4 Apr 2010, Mikle wrote:

> Hello, list! I've got some strange problem with one-disk zfs-pool:
> read/write performance for the files on the fs (dd if=/dev/zero
> of=/mountpoint/file bs=4M count=100) gives me only 2 MB/s, while reading
> from the disk (dd if=/dev/disk of=/dev/zero bs=4M count=100) gives me
> ~70MB/s. pool is about 80% full; PC with the pool has 2GB of ram (1.5 of
> which is free); i've done no tuning in loader.conf and sysctl.conf for
> zfs. In dmesg there is no error-messages related to the disk (dmesg|grep
> ^ad12); s.m.a.r.t. seems OK. Some time ago disk was OK, nothing in
> software/hardware has changed from that day. Any ideas what could have
> happen to the disk?

Has it ever been close to 100% full? How long has it been 80% full and
what kind of files are on it, size wise?