Suspeito que você provavelmente esteja enfrentando o problema que chamarei de "setores de 512 bytes não são setores de 4K". Enfim, google on gnop, setor 4K, WD Green e eu suspeito que você vai encontrar a correção. Eu soube disso pela primeira vez em este site , e foi muito informativo sobre esse assunto, bem como uma variedade de outros ajustes para o FreeBSD e ZFS. Boa sorte!
Editar: para citar o site vinculado:
... Finally, I came across references to problems with Western Digital’s 1.5 TB (WE15EADS) Green drives that I am using.
The drives have a 4KB physical sector but report 512 Bytes to the BIOS. So performance drops off on really big writes because zfs on FreeBSD sends 4KB of data to the drive as 8 separate writes of 512 bytes, which requires the firmware in the drive to increase its work load by an estimated factor of 60 (1st 512 Bytes - write 4KB, 2nd 512 Bytes, read 4K, write 4K, ..., 8th 512 Bytes, read 4K, write 4K -- so 4KB of writes become 4KB write + (4KB read + 4KB write)X(4KB/512Bytes - 1) = 60. The drives built in 32 MB cache helps until it fills and the zfs arc kicks in and then the arc begins to fill. So all in all, no big deal right?
Actually it is a very big deal if you are writing files to zfs that are larger than your arc plus the size of the buffer on the drive. ...