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SubjectRe: 2.5.62-mm2 slow file system writes across multiple disks
On Mon, Feb 24, 2003 at 08:43:21PM -0800, Andrew Morton wrote:
> Patrick Mansfield <patmans@us.ibm.com> wrote:

> > I moved to 2.5.62-mm3 [I had to drop back to qlogicisp for my boot disk,
> > and run the feral drver as a module in order to boot without hanging], and
> > ran write-and-fsync with -f, with and without -o (O_DIRECT).

> Is this using an enormous request queue or really deep TCQ or something?
> I always turn TCQ off, stupid noxious thing it is.

Yes - pretty high, the feral driver is defaulting to 63 (comments there
say "FIX LATER"). Changing it to 8 gave much improved performance.

I'm on a slightly different kernel, anyway:

10 fsync writes of 200 mb to 10 separate disks, queue depth (TCQ) of 63 gave:

0.05user 37.14system 1:09.39elapsed 53%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (1986major+3228minor)pagefaults 0swaps

10 fsync writes of 200 mb to 10 separate disks, queue depth of 8 gave:

0.05user 38.56system 0:32.71elapsed 118%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (1986major+3228minor)pagefaults 0swaps

The total time dropped by almost half. vmstat numbers were much smoother.

It would be nice if a larger queue depth did not kill performance.

The larger queue depths can be nice for disk arrays with lots of cache and
(more) random IO patterns.

-- Patrick Mansfield
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