Strange ZFS behaviour with transmission

[Alexey Tarasov]

Hello.

I have a machine with 4x1.5 Tb SATA disks (WDC WD15EARS). HDD's are in AHCI mode (ada).
I use root-on-ZFS with GPT and RAIDZ.

I have the following problem: when I run transmission torrent client on this server, the FS works very slow, all processes hangs in on of the zfs, zio, tx states.
According to gstat disk load changes very fast from 0 to 100%. After some activity of all disks there is 2-3 seconds of 100% activity of the 4-th HDD. During this activity FS is inaccessible. I have already replaced this disk with new one and there were no effect.
Is this a bug or some wrong configuration?

/boot/loader.conf:
zfs_load="YES"
ahci_load="YES"
vm.kmem_size=2G
vm.kmem_size_max=2G
vfs.root.mountfrom="zfs:zroot"
vfs.zfs.prefetch_disable=1

zpool status
 pool: zroot
state: ONLINE
scrub: none requested
config:

	NAME           STATE     READ WRITE CKSUM
	zroot          ONLINE       0     0     0
	  raidz1       ONLINE       0     0     0
	    gpt/disk0  ONLINE       0     0     0
	    gpt/disk1  ONLINE       0     0     0
	    gpt/disk2  ONLINE       0     0     0
	    gpt/disk3  ONLINE       0     0     0

FreeBSD st4 8.0-STABLE FreeBSD 8.0-STABLE #0: Mon Mar  1 17:23:20 UTC 2010     root@:/usr/obj/usr/src/sys/GENERIC  amd64


--
Alexey Tarasov