SAS5IR performance issue with Dell 860

Tom Judge tom at
Sun Nov 25 08:02:55 PST 2007

Espen Tagestad wrote:
> Hi,
> We recently bought 3 new Dell 860 servers with the onboard SAS5/IR SATA 
> RAID-controller. They seem to be quite well spec'ed servers with 
> management and everything - but I am experiencing av major performance 
> issue with the disc i/o. On write I get at max 7-8MB/sec, while read 
> gives a bit more (11MB/sec). I tried first with 6.2-RELEASE, and then 
> upgraded to 6.3-PRERELEASE without any better results.
> I am aware of some discussion around this issue on these two maillists 
> in the spring earlier on this year, but I have not been able to find any 
> good resolution. My old firewall/router at home equipped with a 733Mhz 
> Pentium 3 processor and a old 40GB IDE harddrive made around year 2001 
> performe better. Is there anybody out there with the same problem who 
> has solved this issue? Could it be that this is solved in 7.0?
> Thanks in advance.

If you have sata disks on this controller you will need to run RELENG_6 
dated after 2007-06-05 21:32:57.

You will also need to set the hw.mpt.enable_sata_wc sysctl in loader.conf.

Here is Scott Long's commit log of the changes that 'fix' this issue.


Commit Message:

scottl      2007-06-03 23:13:05 UTC

    FreeBSD src repository

    Modified files:
      sys/dev/mpt          mpt.c mpt.h mpt_cam.c
            Add support for reading extended configuration pages.
            Do a top level topology scan on the SAS controller.  If any
SATA device are discovered in this scan, send a passthrough FIS to set
the write cache.  This is controllable through the following tunable at
            -1 = Do not configure, use the controller default
             0 = Disable the write cache
             1 = Enable the write cache

            The default is -1.  This tunable is just a hack and may be
            deprecated in the future.

Turning on the write cache alleviates the write performance problems
with SATA that many people have observed.  It is not recommend for those
who value data reliability!  I cannot stress this strongly enough.
However,  it is useful in certain circumstances, and it brings the
performence in line with what a generic SATA controller running under
the FreeBSD ATA driver provides (and the ATA driver has had the WC
enabled by default for years).

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