PowerEdge 1435 and Linux 2.6.20: disk I/O problems

Hi all

I've a PE1435 with a SATA disk and I'm experiencing TREMENDOUS problems with disk access. I don't know if it's due to broken hardware or due to bugged kernel but, this is what I got when accessing the disk:

Jun 11 18:12:27 static2 ata4: SATA max UDMA/133 cmd 0xFFFFC2000001C300 ctl 0xFFFFC2000001C320 bmdma 0xFFFFC2000001C330 irq 10
Jun 11 18:12:27 static2 scsi0 : sata_svw
Jun 11 18:12:27 static2 ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
Jun 11 18:12:27 static2 ata1.00: ATA-7, max UDMA/133, 312500000 sectors: LBA48 NCQ (depth 0/32)
Jun 11 18:12:27 static2 ata1.00: ata1: dev 0 multi count 8
Jun 11 18:12:27 static2 ata1.00: configured for UDMA/133
Jun 11 18:12:27 static2 scsi1 : sata_svw
Jun 11 18:12:27 static2 ata2: SATA link down (SStatus 4 SControl 300)
Jun 11 18:12:27 static2 ATA: abnormal status 0x7F on port 0xFFFFC2000001C11C
Jun 11 18:12:27 static2 scsi2 : sata_svw
Jun 11 18:12:27 static2 ata3: SATA link down (SStatus 4 SControl 300)
Jun 11 18:12:27 static2 ATA: abnormal status 0x7F on port 0xFFFFC2000001C21C
Jun 11 18:12:27 static2 scsi3 : sata_svw
Jun 11 18:12:27 static2 ata4: SATA link down (SStatus 4 SControl 300)
Jun 11 18:12:27 static2 ATA: abnormal status 0x7F on port 0xFFFFC2000001C31C
Jun 11 18:12:27 static2 scsi 0:0:0:0: Direct-Access ATA ST3160812AS 3.AD PQ: 0 ANSI: 5


I've already changed the disk and the problem remains exactly the same. Moreover, a dd between two disks attached to the same controller (channel A and B) gives a ridiculous 5MB/sec transfer rate.

Does anyone experience the same problem? I still haven't tried with another kernel so I cannot clearly isolate the problem.

Exact kernel version:
2.6.20-gentoo-r8 #3 SMP Wed May 23 11:16:09 CEST 2007 x86_64 Dual-Core AMD Opteron(tm) Processor 2212 AuthenticAMD GNU/Linux