Dell/Perc5 raid/MPT SAS Integrated Raid Write Performance

Filip Palian filip.palian at
Wed Jun 13 09:06:23 UTC 2007

Tom Judge pisze:
> Filip Palian wrote:
>> Hi folks,
>> I've also encountered problems with write performnce on PE860, below are
>> some details.
> I believe that a fix for this went into RELENG_6 this week.
> Scott Long MFC'd the following fix at 2007-06-05 21:32:57.
> Tom

Great thing that sombody cares.

> 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
>   Log:
>   mpt.c:
>   mpt.h:
>           Add support for reading extended configuration pages.
>   mpt_cam.c:
>           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
> boot:
>     hw.mpt.enable_sata_wc:
>           -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.

Unfortunatly I couldn't use this fix because disk performance is crucial
for me and i could not depend on the "hack" which 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).

Just for curiosity I've tried to run FreeBSD 6.2 in Xen and KVM with
Gentoo as a host system. Under Xen nothing could be done because of "BTX
halted" error with the same values as described in the bug
(in my example it was an IDE cdrom). But happily KVM did the work. I've
intsalled FreeBSD and run blogbench and guess what... The results were:

Final score for writes: 84
Final score for reads : 13636

Summing up, I'll wait for 7.0 release and then I'll put my hands on it.

Filip Palian

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