very slow write to AHCI device

[Daniel Pocock <daniel@pocock.com.au>]

Hi,

I'm testing a problem with the following combination:
- SB700/SB800 type controller in AHCI mode (in a HP Microserver N36L)
- Seagate Barracuda 1TB 7k2 drive with NCQ

I've got a fresh 256MB partition, formatted ext4, mounted
barrier=1,data=ordered, write-cache enabled (hdparm -W 1 /dev/sdb) and
shared over NFS.

When the NFS client writes,
- unpacking a source tarball, many small files, iostat reports speeds
under 500kBytes/sec
- dd conv=fsync, iostat reports about 50MB/sec

If I set up a USB disk on the same box, with a partition formatted
exactly the same way, the iostat reports the write speed (unpacking the
same tarball) is over 5MBytes/sec - not so fast, but 10 times faster
than the AHCI device.

Is there anything I can do to troubleshoot this issue?

Or should I just stop using this onboard SATA and go to something like
an Adaptec 1405?

Regards,

Daniel