Slow write performance with mpt2sas

I've noticed that my SAS9212-4i4e is having write issues. I have a raid5 of 4 2tb seagate green drives and a raid5 of 3 300gb seagate 15k sas drives. Write performance on the green drives is fairly poor. The riad4 of 4 disks has a write performance of about 6-10mb/sec. My onboard sas controller (1068e chip IT mode) does not have this issue (I'm using raid6 on 8 of the same model green drives).

Write performance on the 15k drives in raid 5 was 140mb/sec (4gb volume).

Here's some information from lsi's sas2flash util:

        Controller                     : SAS2008(B1)   
        PCI Address                    : 00:03:00:00
        SAS Address                    : 500605b-0-01d0-ab00
        NVDATA Version (Default)       : 0a.03.00.02
        NVDATA Version (Persistent)    : 0a.03.00.02
        Firmware Product ID            : 0x2713 
        Firmware Version               : 11.00.00.00
        NVDATA Vendor                  : LSI
        NVDATA Product ID              : SAS9212-4i4e
        BIOS Version                   : 07.21.00.00
        UEFI BSD Version               : N/A
        FCODE Version                  : N/A
        Board Name                     : SAS9212-4i4e
        Board Assembly                 : H3-25134-00A
        Board Tracer Number            : P131430410

I'm using stock kernel 2.6.39 compiled for 64-bit. This sas controller has IR firmware.

Relevent information from dmesg:

[    1.585459] mpt2sas version 08.100.00.00 loaded
[    1.585564] scsi0 : Fusion MPT SAS Host
[    1.585772] mpt2sas 0000:03:00.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
[    1.585846] mpt2sas 0000:03:00.0: setting latency timer to 64
[    1.585849] mpt2sas0: 64 BIT PCI BUS DMA ADDRESSING SUPPORTED, total mem (12301640 kB)
[    1.585966] mpt2sas0: IO-APIC enabled: IRQ 16
[    1.586027] mpt2sas0: iomem(0x00000000fac3c000), mapped(0xffffc90000020000), size(16384)
[    1.586110] mpt2sas0: ioport(0x000000000000a000), size(256)
[    1.872511] mpt2sas0: sending diag reset !!
[    2.355485] Refined TSC clocksource calibration: 2133.333 MHz.
[    2.355561] Switching to clocksource tsc
[    2.974166] mpt2sas0: diag reset: SUCCESS
[    3.121510] mpt2sas0: Allocated physical memory: size(3999 kB)
[    3.121574] mpt2sas0: Current Controller Queue Depth(1753), Max Controller Queue Depth(2000)
[    3.121659] mpt2sas0: Scatter Gather Elements per IO(128)
[    3.353750] mpt2sas0: LSISAS2008: FWVersion(11.00.00.00), ChipRevision(0x02), BiosVersion(07.21.00.00)
[    3.353848] mpt2sas0: Protocol=(Initiator), Capabilities=(Raid,TLR,EEDP,Snapshot Buffer,Diag Trace Buffer,Task Set Full,NCQ)
[    3.354367] mpt2sas0: sending port enable !!
[    4.888265] mpt2sas0: host_add: handle(0x0001), sas_addr(0x500605b001d0ab00), phys(8)
[   10.998203] mpt2sas0: port enable: SUCCESS

Any suggestions?

PS. I am on the list. I do prefer courtesy CC.

 Microsoft has beaten Volkswagen's world record.  Volkswagen only created 22
 million bugs.

Sorry, this appears to be a KVM issue. I had forgotten that I was using this under KVM. Adding cache=writeback fixed my problems (Also note, my load before this went to >35 when writing, now it's no higher than about 9)