Hey Doug,
I'm trying to get the patch to work and not having much luck.
I'm guessing that you're using the generic CentOS 7 kernel (3.10). I'm using a 3.18.4 kernel, so there were a few changes I had to make to get the patch to apply. But they weren't significant as far as I can tell, and shouldn't have caused the FastWrite code to be ignored.
There must be additional changes in the kernel that are necessary. When I create the special case MDRaid, and then use the special case IO pattern, I see only the debug messages indicating that the FastWrite code was ignored.
I've added more debugging code to try to understand what's going on. It turns out that no matter what I do, no matter how I configure the MDRaid, and no matter what IO pattern I use, the size of the IO is such that it doesn't conform to the criteria required to call FastWrite. It seems that in the 3.18 .4 kernel, IO's are broken up before calling MDRaid's make_request function. There doesn't seem to be an obvious relationship between the size being passed in, and the chunk size that's configured by mdadm.
At first, it appeared that it was the minimum IO size being used. But then I tried setting the minimum IO and optimal IO to the same thing. The resulting IO's are now 1/2 the size of the minimum IO.... which is not the size of the optimal IO (which is the chunk size * number of data disks).
Bob Kierski
Senior Storage Performance Engineer
Cray Inc.
380 Jackson Street
Suite 210
St. Paul, MN 55101
Tele: 651-967-9590
Fax: 651-605-9001
Cell: 651-890-7461
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