12x performance drop on md/linux+sw raid1 due to barriers [xfs]

[Bill Davidsen]

Justin Piszcz wrote:
> Someone should write a document with XFS and barrier support, if I 
> recall,
> in the past, they never worked right on raid1 or raid5 devices, but it
> appears now they they work on RAID1, which slows down performance ~12 
> times!!
>
I would expect you, as an experienced tester, to have done this 
measurement more rigorously!
I don't think it means much if this is what you did.

> l1:~# /usr/bin/time tar xf linux-2.6.27.7.tar 0.15user 1.54system 
> 0:13.18elapsed 12%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (0major+325minor)pagefaults 0swaps
> l1:~#
>
> l1:~# /usr/bin/time tar xf linux-2.6.27.7.tar
> 0.14user 1.66system 2:39.68elapsed 1%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 0inputs+0outputs (0major+324minor)pagefaults 0swaps
> l1:~#
>
Before doing any disk test you need to start by dropping cache, to be 
sure the appropriate reproducible things happen. And in doing a timing 
test, you need to end with a sync for the same reason.

So:
 echo 1 >/proc/sys/vm/drop_caches
 time bash -c "YOUR TEST; sync"

This will give you a fair shot at being able to reproduce the results, 
done on an otherwise unloaded system.

-- 
Bill Davidsen <davidsen at tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark