Re: Dell/Perc5 raid/MPT SAS Integrated Raid Write Performance

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
"http://lists.freebsd.org/pipermail/freebsd-bugs/2006-November/020906.html";
(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.

Regards,
Filip Palian
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