RAID 5 Performance Issue with SA-P600

[nomadsole]

We have 4 Seagate SATA II drives (ST3320620AS) attached to SA-P600.

The write performance sucks when using RAID 5. IOMeter with 32K; 0% Read, 0% Random shows a stable write rate of 20MB/s or so.

The read rate looks ok, but not good enough. IOMeter with 32K; 100% Read, 0% Random shows a stable read rate of 66MB/s with 4 physical drives.

Btw, the stripe size is the defaul 64K stripe size. All windows partitions are aligned to 1MB. NTFS allocation unit (cluster) size is 64K.

Running ADU shows:

1. the transfer speed is only 1.5Gbps.
2. NCQ is not supported.
3. the write caches on the physical drives are disabled but changable. But executing command ctrl slot=5 modify dwc=enable forced returns an error, saying it can be done with the current configuration.

Does anyone know what causes the problem?